Somatostatin derivatives and their radiolabelled products

ABSTRACT

This invention relates to therapeutic reagents and peptides, radiodiagnostic reagents and peptides, and methods for producing labelled radiodiagnostic agents. Specifically, the invention relates to linear peptide derivatives and analogs of somatostatin, and embodiments of such peptides radiolabelled with a radioisotope, as well as methods and kits for making, radiolabelling and using such peptides for radiodiagnostic and radiotherapeutic purposes. The invention specifically relates to linear peptide derivatives and analogues of somatostatin radiolabelled with technetium-99m and uses thereof as scintigraphic imaging agents. The invention also specicically relates to linear peptide derivatives and analogues of somatostatin radiolabelled with cytotoxic radioisotopes such as rhenium-186 ( 186 Re) and rhenium-188 ( 188 Re) for use as radiotherapeutic agents. Methods and kits for making, radiolabelling and using such peptides diagnostically and therapeutically in a mammalian body are also provided.

This application is a 371 of PCT/US94/08335 filed Jul. 21, 1994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to therapeutic agents and peptides, radiotherapeutic agents and peptides, radiodiagnostic agents and peptides, and methods for producing such labeled radiodiagnostic and radiotherapeutic agents. Specifically, the invention relates to linear peptide derivatives and analogues of somatostatin, and embodiments of such peptides labeled with gamma radiation-emitting radioisotopes such as technetium-99m (Tc-99m), as well as methods and kits for making, radiolabeling and using such peptides to image sites in a mammalian body. The invention also relates to linear peptide derivatives and analogues of somatostatin labeled with cytotoxic radioisotopes such as rhenium- 186 (¹⁸⁶Re) and rhenium- 188 (¹⁸⁸Re), and methods and kits for making, radiolabeling and using such peptides therapeutically in a mammalian body.

2. Description of the Prior Art

Somatostatin is a tetradecapeptide that is endogenously produced by the hypothalamus and pancreas in humans and other mammals. The peptide has the formula:

(Single letter abbreviations for amino acids can be found in G. Zubay, Biochemistry (2d ed.), 1988, (MacMillan Publishing: New York), p.33). This peptide exerts a wide variety of biological effects in vivo. It is known to act physiologically on the central nervous system, the hypothalamus, the pancreas, and the gastrointestinal tract.

Somatostatin inhibits the release of insulin and glucagon from the pancreas, inhibits growth hormone release from the hypothalamus, and reduces gastric secretions. Thus, somatostatin has clinical and therapeutic applications for the alleviation of a number of ailments and diseases, both in humans and other animals. Native somatostatin is of limited utility, however, due to its short half-life in vivo, where it is rapidly degraded by peptidases. For this reason, somatostatin analogues having improved in vivo stability have been developed in the prior art.

Freidinger, U.S. Pat. No. 4,235,886 disclose cyclic hexapeptide somatostatin analogues useful in the treatment of a number of diseases in humans.

Coy and Murphy, U.S. Pat. No. 4,485,101 disclose synthetic dodecapeptide somatostatin analogues.

Freidinger, U.S. Pat. No. 4,611,054 disclose cyclic hexapeptide somatostatin analogues useful in the treatment of a number of diseases in humans.

Nutt, U.S. Pat. No. 4,612,366 disclose cyclic hexapeptide somatostatin analogues useful in the treatment of a number of diseases in humans.

Coy et al., U.S. Pat. No. 4,853,371 disclose synthetic octapeptide somatostatin analogues.

Coy and Murphy, U.S. Pat. No. 4,871,717 disclose synthetic heptapeptide somatostatin analogues.

Coy et al., U.S. Pat. No. 4,904,642 disclose synthetic octapeptide somatostatin analogues.

Taylor et al., U.S. Pat. No. 5,073,541 disclose a method of treating small cell lung cancer.

Brady, European Patent Application No. 83111747.8 discloses dicyclic hexapeptide somatostatin analogues useful in the treatment of a number of human diseases.

Bauer et al., European Patent Application No. 85810617.2 disclose somatostatin derivatives useful in the treatment of a number of human diseases.

Eck and Moreau, European Patent Application No. 90302760.5 disclose therapeutic octapeptide somatostatin analogues.

Coy and Murphy, European Patent Application Serial No. 90304551.6 disclose linear somatostatin analogues.

Coy and Murphy, International Patent Application Serial No. PCT/US90/07074 disclose somatostatin analogues for therapeutic uses.

Schally et al., European Patent Application Serial No. EPA 911048445.2 disclose cyclic peptides for therapeutic use.

Bodgen and Moreau, International Patent Application Serial No. PCT/US92/01027 disclose compositions and methods for treating proliferative skin disease.

Somatostatin exerts it effects by binding to specific receptors expressed at the cell surface of cells comprising the central nervous system, the hypothalamus, the pancreas, and the gastrointestinal tract. These high-affinity somatostatin binding sites have been found to be abundantly expressed at the cell surface of most endocrine-active tumors arising from these tissues. Expression of high-affinity binding sites for somatostatin is a marker for these tumor cells, and specific binding with somatostatin can be exploited to locate and identify tumor cells in vivo.

Methods for radiolabeling somatostatin analogues that have been modified so as to contain a tyrosine amino acid (Tyr or Y) are known in the prior art.

Albert et al., UK Patent Application 8927255.3 disclose radioimaging using somatostatin derivatives such as octreotide labeled with ¹²³I.

Bakker et al., 1990, J. Nucl. Med. 31: 1501-1509 describe radioactive iodination of a somatostatin analog and its usefulness in detecting tumors in vivo.

Bakker et al., 1991, J. Nucl. Med. 32: 1184-1189 teach the usefulness of radiolabeled somatostatin for radioimaging in vivo.

Bomanji et al., 1992, J. Nucl. Med. 33: 1121-1124 describe the use of iodinated (Tyr-3) octreotide for imaging metastatic carcinoid tumors.

Alternatively, methods for radiolabeling somatostatin by covalently modifying the peptide to contain a radionuclide-chelating group have been disclosed in the prior art.

Albert et al., UK Patent Application 8927255.3 disclose radioimaging using somatostatin derivatives such as octreotide labeled with ¹¹¹In via a chelating group bound to the amino-terminus.

Albert et al., European Patent Application No. WO 91/01144 disclose radioimaging using radiolabeled peptides related to growth factors, hormones, interferons and cytokines and comprised of a specific recognition peptide covalently linked to a radionuclide chelating group.

Albert et al., European Patent Application No. 92810381.1 disclose somatostatin peptides having amino-terminally linked chelators.

Faglia et al., 1991, J. Clin. Endocrinol. Metab. 73: 850-856 describe the detection of somatostatin receptors in patients.

Kwekkeboom et al., 1991, J. Nucl. Med. 32: 981 Abstract #305 relates to radiolabeling somatostatin analogues with ¹¹¹In.

Albert et al., 1991, Abstract LM10, 12th American Peptide Symposium: 1991 describe uses for ¹¹¹In-labeled diethylene-triaminopentaacetic acid-derivatized somatostatin analogues.

Krenning et al., 1992, J. Nucl. Med. 33: 652-658 describe clinical scintigraphy using (¹¹¹In)(DTPA)octreotide.

These methods can be readily adapted to enable detection of tumor cells in vivo by radioimaging, based on the expression of high affinity binding sites for somatostatin on tumor cells. Radionuclides which emit gamma radiation can be readily detected by scintigraphy after injection into a human or an animal. A variety of radionuclides are known to be useful for radioimaging, including ⁶⁷Ga, ⁶⁸Ga, ^(99m)Tc (Tc-99m), ¹¹¹In, ¹²³I or ¹²⁵I. The sensitivity of imaging methods using radioactively-labeled peptides is much higher than other techniques known in the art, since the specific binding of the radioactive peptide concentrates the radioactive signal over the cells of interest, for example, tumor cells. This is particularly important for endocrine-active gastrointestinal tumors, which are usually small, slow-growing and difficult to detect by conventional methods. Labeling with technetium-99m (Tc-99m) is advantageous because the nuclear and radioactive properties of this isotope make it an ideal scintigraphic imaging agent. Tc-99m has a single photon energy of 140 keV and a radioactive half-life of about 6 hours, and is readily available from a ⁹⁹Mo-^(99m)Tc generator. Other radionuclides have effective half-lives which are much longer (for example, ¹¹¹In, which has a half-life of 60-70 h) or are toxic (for example, ¹²⁵I). Although Tc-99m is an ideal radiolabeling reagent, it has not been widely used in the art prior to the present invention (see, for example, Lamberts, J. Nucl. Med. 32: 1189-1191 (1991)).

Somatostatin and radiolabeled somatostatin analogues can also be used therapeutically. For these applications, cytotoxic radioisotopes are advantageous, such as scandium47, copper-67, gallium-72, yttrium-90, iodine-125, iodine-131, samarium-153, gadolinium-159, dysprosium-165, holmium-166, ytterbium-175, lutetium-177, rhenium-186, rhenium-188, astatine-211, and bismuth-212. The rhenium isotopes ¹⁸⁶Re and ¹⁸⁸Re are particularly advantageous.

The use of chelating agents for radiolabeling proteins are known in the prior art, and methods for labeling peptides Tc-99m are disclosed in co-pending U.S. patent applications Ser. No. 07/653,012, now abandoned, a divisional of which issued as U.S. Pat. No. 5,654,272; U.S. Ser. No. 07/757,470, now U.S. Pat. No. 5,225,180; U.S. Ser. No. 07/807,062, now U.S. Pat. No. 5,443,815; U.S. Ser. No. 07/851,074, now abandoned, a divisional of which issued as U.S. Pat. No. 5,711,931; U.S. Ser. No. 07/871,282, a divisional of which issued as U.S. Pat. No. 5,780,007; U.S. Ser. No. 07/886,752, now abandoned, a divisional of which issued as U.S. Pat. No. 5,736,122; U.S. Ser. No. 07/893,981, now U.S. Pat. No. 5,508,020; U.S. Ser. No. 07/955,466; U.S. Ser. No. 07/977,628, now U.S. Pat. No. 5,405,597; U.S. Ser. No. 08/019,864, now U.S. Pat. No. 5,552,525; U.S. Ser. No. 08/044,825, now abandoned, which issued as U.S. Pat. No. 5,645,815; and U.S. Ser. No. 08/073,577, now U.S. Pat. No. 5,561,220; U.S. Ser. No. 08/092,355; U.S. Ser. No. 08/095,760, now U.S. Pat. No. 5,620,675; U.S. Ser. No. 08/098,206, now abandoned, a divisional of which issued as U.S. Pat. No. 5,770,179; U.S. Ser. No. 08/210,822, now abandoned; U.S. Ser. Nos. 08/236,402; 08/241,625, now U.S. Pat. No. 5,783,170; allowed U.S. Ser. No. 08/244,336, allowed U.S. Ser. No. 08/253,317, and 08/253,678; and PCT International Applications PCT/US92/00757, PCT/US92/10716, PCT/US93/02320, PCT/US93/03687, PCT/US93/04794, PCT/US93/06029, PCT/US93/09387, PCT/US94/01894, PCT/US94/05895, and PCT/US94/06274, which are hereby incorporated by reference.

Fritzberg, U.S. Pat. No. 4,444,690 describes a series of technetium-chelating agents based on 2,3-bis(mercaptoacetamido) propanoate.

Gansow et al., U.S. Pat. No. 4,472,509 teach methods of manufacturing and purifying Tc-99m chelate-conjugated monoclonal antibodies.

Reno and Bottino, European Patent Application 87300426.1 disclose radiolabeling antibodies with Tc-99m.

Pak et al., European Patent Application No. WO 88/07382 disclose a method for labeling antibodies with Tc-99m.

Cox, International Patent Application No. PCT/US92/04559 discloses radiolabeled somatostatin derivatives containing two cysteine residues.

Rhodes, 1974, Sem. Nucl. Med. 4: 281-293 teach the labeling of human serum albumin with technetium-99m.

Khaw et al., 1982, J. Nucl. Med. 23: 1011-1019 disclose methods for labeling biologically active macromolecules with Tc-99m.

Byrne and Tolman, supra, disclose a bifunctional thiolactone chelating agent for coupling Tc-99m to biological molecules.

Cox et al., 1991, Abstract, 7th International Symposium on Radiopharmacology, p. 16, disclose the use of, Tc-99m-, ¹³¹I- and ¹¹¹In-labeled somatostatin analogues in radiolocalization of endocrine tumors in vivo by scintigraphy.

Methods for directly labeling somatostatin, derivatives of somatostatin, analogues of somatostatin or peptides that bind to the somatostatin receptor and contain at least 2 cysteine residues that form a disulfide or wherein the disulfide is reduced to the sulfhydryl form, are disclosed in co-pending U.S. patent application Ser. No. 07/807,062, now U.S. Pat. No. 5,225,180, issued Jul. 6, 1993 which is hereby incorporated by reference.

There remains a need for synthetic (to make routine manufacture practicable and to ease regulatory acceptance) somatostatin analogues having increased in vivo stability, to be used therapeutically, as scintigraphic agents when radiolabeled with Tc-99m or other detectable radioisotopes for use in imaging tumors in vivo, and as radiotherapeutic agents when radiolabeled with a cytotoxic radioisotope such as rhenium-188. Small synthetic somatostatin analogues are provided by this invention that specifically fulfill this need.

SUMMARY OF THE INVENTION

The present invention provides somatostatin analogues that are linear peptides for therapeutic applications, including radiotherapeutic applications, and diagnostic applications, including radiodiagnostic applications, in particular scintigraphic imaging applications. Distinct from native somatostatin and somatostatin analogues known in the prior art, the linear peptides of the invention are not constrained within a cyclic structure. The invention also provides linear peptide reagents comprised of the linear peptide somatostatin analogues of the invention, wherein such peptides are covalently linked to a radiolabel-binding moiety. The invention provides such linear peptides, linear peptide reagents and radiolabeled linear peptide reagents that are scintigraphic imaging agents, radiodiagnostic agents and radiotherapeutic agents. Scintigraphic imaging agents of the invention comprise linear peptide reagents radiolabeled with a radioisotope, preferably technetium-99m. Radiotherapeutic agents of the invention comprise linear peptide reagents radiolabeled with a cytotoxic radioisotope, preferably rhenium-186 or rhenium-188. Methods for making and using such linear peptides, linear peptide reagents and radiolabeled embodiments thereof are also provided.

The present invention also provides scintigraphic imaging agents comprised of a linear peptide that is a somatostatin analogue and that is labeled with iodine-123, iodine-125 or iodine-131. Similarly, the invention provides alternative embodiments of the linear somatostatin peptide analogues radiolabeled with iodine-125, iodine-131 or astatine-211 for use as therapeutic agents.

The somatostatin analogues provided by the invention are somatostatin-receptor binding peptides having the following formula:

X¹-A¹A²-B¹B²B³B⁴-C¹C²-X²  Formula II

wherein X¹ is a hydrophilic moiety which is not greater than 1500 Daltons in formula weight; A¹, A² and C¹ are each independently a lipophilic D- or L-amino acid, S-alkylated cysteine, penicillamine (Pen), homocysteine (Hcy) or homohomocysteine (Hhc; 3-mercaptopropyl) glycine; B¹ is D- or L-Phe, or D- or L-Tyr, or D- or L-2-naphthylalanine (Nal), or 2-amino-indane-2-carboxylic acid (Ain) or substituted derivatives thereof; B² is D- or L-Trp or substituted derivatives thereof; B³ is D- or L-Lys, or homolysine (Hly), 4-amino-cyclohexylalanine (Achxa), 4-aminomethyl-phenylalanine (Amf), S-(2-aminoethyl)cysteine (Aec), S-(3-aminopropyl)cysteine (Apc), O-(2-aminoethyl) serine (Aes), O-(3-aminopropyl)serine (Aps) or substituted derivatives thereof; B⁴ is Thr, Ser, Val, Phe, Leu, Ile, 2-amino-isobutyric acid (Aib), 2-aminobutyric acid (Abu), norvaline (Nva), or norleucine (Nle); C² is D- or L-Thr, Ser, Val, Phe, Ile, Abu, Nle, Leu, Nva, Nal or Aib or substituted derivatives thereof; X² is a hydrophilic moiety which is not more than 1500 Daltons in formula weight. In a preferred embodiment, X¹ is a hydrophilic moiety that comprises an amino acid, or a peptide having an amino acid sequence of no more than 10 residues, or a monosaccharide, or an oligosaccharide comprising 10 or fewer saccharide units, or a poly(N-carboxyalkyl)amine, or a polyoxyanion. In another preferred embodiment, X² is a hydrophilic moiety that comprises a poly(N-carboxyalkyl)amine or polyoxyanion, or an amino acid, or a peptide having an amino acid sequence of no more than 10 residues (including peptides wherein the carboxyl group of the carboxyl-terminal amino acid is reduced to an alcohol), or a monosaccharide or an oligosaccharide comprising 10 or fewer saccharide units. In another preferred embodiment, B¹ is phenylalanine or tyrosine, B² is tryptophan, most preferably D-tryptophan, B³ is lysine and B⁴ is threonine or valine.

The invention also provides linear peptide reagents comprising a linear peptide of Formula II covalently linked to a radiolabel-binding moiety, wherein X¹ is H, lower alkyl or substituted alkyl, aryl or substituted aryl, alkanoyl or substituted alkanoyl, aroyl or substituted aroyl, or a hydrophilic moiety which is not greater than 1500 Daltons in formula weight; A¹, A² and C¹ are each independently a lipophilic D- or L-amino acid, S-alkylated cysteine, penicillamine (Pen), homocysteine (Hcy) or homohomocysteine (Hhc; 3-mercaptopropyl) glycine; B¹ is D- or L-Phe, or D- or L-Tyr, or D- or L-2-naphthylalanine (Nal), or 2-amino-indane-2-carboxylic acid (Ain) or substituted derivatives thereof; B² is D- or L-Trp or substituted derivatives thereof; B³ is D- or L-Lys, or homolysine (Hly), 4-amino-cyclohexylalanine (Achxa), 4-aminomethyl-phenylalanine (Amf), S-(2-aminoethyl)cysteine (Aec), S-(3-aminopropyl)cysteine (Apc), O-(2-aminoethyl) serine (Aes), O-(3-aminopropyl)serine (Aps) or substituted derivatives thereof; B⁴ is Thr, Ser, Val, Phe, Leu, Ile, 2-amino-isobutyric acid (Aib), 2-aminobutyric acid (Abu), norvaline (Nva), or norleucine (Nle); C² is D- or L-Thr, Ser, Val, Phe, Ile, Abu, Nle, Leu, Nva, Nal or Aib or substituted derivatives thereof; X² is -COOR⁹, -CH₂OH, CH₂COOR⁹, or -CON(R⁹)₂, where each R⁹ is independently H, lower linear or cyclic alkyl or substituted derivatives thereof, or substituted with a hydrophilic moiety which is not more than 1500 Daltons in formula weight. In a preferred embodiment, when X¹ is a hydrophilic moiety that moiety comprises an amino acid, or a peptide having an amino acid sequence of no more than 10 residues, or a monosaccharide, or an oligosaccharide comprising 10 or fewer saccharide units, or a poly(N-carboxyalkyl)amine, or a polyoxyanion. In another preferred embodiment, when X² is a hydrophilic moiety that moiety comprises a poly(N-carboxyalkyl)amine or polyoxyanion, or an amino acid, or a peptide having an amino acid sequence of no more than 10 residues (including peptides wherein the carboxyl group of the carboxyl-terminal amino acid is reduced to an alcohol), or a monosaccharide or an oligosaccharide comprising 10 or fewer saccharide units. In another preferred embodiment, B¹ is phenylalanine or tyrosine, B² is tryptophan, most preferably D-tryptophan, B³ is lysine and B⁴ is threonine or valine.

The present invention provides peptides that are linear somatostatin peptide analogues as described herein having increased in vivo stability compared with native somatostatin, and that are therapeutically useful in the alleviation of diseases or other ailments in humans or other animals.

The invention also provides scintigraphic imaging agents comprising the linear peptide reagents of the invention wherein the radiolabel-binding moiety is stably complexed with a radioisotope. In one such embodiment is provided a scintigraphic imaging agent wherein the linear somatostatin peptide analogue reagents of the invention are radiolabeled with technetium-99m. In other embodiments of the scintigraphic imaging agents of the invention the radioisotope is indium-111 or gallium-68. In still other embodiments, the scintigraphic imaging agents of the invention are linear peptides that are radiolabeled with iodine-123 or iodine-125.

The invention also provides radiotherapeutic agents that are the linear peptide reagents of the invention radiolabeled with a cytotoxic radioisotope that is selected from the group consisting of scandium47, copper-67, gallium-72, yttrium-90, iodine-125, iodine-131, samarium-153, gadolinium-159, dysprosium-165, holmium-166, ytterbium-175, lutetium-177, rhenium-186, rhenium-188, astatine-211 and bismuth-212. In preferred embodiments, the radioisotope is rhenium-186 or rhenium-188. In additional preferred embodiments, the cyclic peptides of the invention are radiolabled with iodine-125, iodine-131 or astatine-211.

In another embodiment, the invention provides therapeutic agents comprising the linear somatostatin analogue peptide reagents of the invention complexed with a non-radioactive metal such as rhenium. Combination embodiments, wherein such a complex is also radiolabeled, either directly or via a radiolabel-binding moiety, are also provided by the invention and are within its scope.

The invention also provides pharmaceutical compositions comprising the somatostatin receptor-binding peptides of the invention in a pharmaceutically acceptable carrier.

The invention also provides a method for alleviating somatostatin-related diseases in animals, preferably humans, comprising administering a therapeutically effective amount of the somatostatin analogues of the invention to the animal. In preferred embodiments, the amount of the somatostatin analogue administered is from about 0.1 to about 50 mg/kg body weight/day.

It is an advantage of the somatostatin analogues provided by this invention that the peptides retain high affinity for somatostatin receptors even though they are linear peptides. As the preferred embodiments lack intramolecular disulfide bonding, the advantageous feature of the linear somatostatin peptide analogues of this invention is that their stability is not dependent on the formation or persistence of intramolecular disulfide bonds. This feature is in turn advantageous because the high affinity of the peptides of this invention for somatostatin receptors is thus not a function of the integrity of labile intramolecular crosslinks such as disulfide bonds. Additionally, the peptide reagents of the invention retain their high affinity for somatostatin receptors after being subjected to radiolabeling via covalently-linked radiolabel binding moieties. In contrast, for example, Tc-99m conjugation to a Tc-99m binding moiety covalently linked to native somatostatin, or to a somatostatin analogue having a disulfide bond, can result in reduction of the disulfide accompanied by a loss of biological activity. Such loss of biological activity can also occur in vivo using native somatostatin, or to any somatostatin analogue having a disulfide bond. The present invention is not subject to similar losses in biological activity in vivo because the somatostatin analogues of the invention are active as linear peptides.

A first aspect of the reagents provided by the invention for preparing radiolabeled agents of the invention are reagents, each comprised of a peptide that is a somatostatin analogue that is covalently linked to a radiolabel-binding moiety having formula:

C(pgp)^(s)-(aa)-C(pgp)^(s)

wherein (pgp)^(s) is H or a thiol protecting group and (aa) is any α- or β-amino acid. In a preferred embodiment, the amino acid is glycine. In another preferred embodiment, the agent is a scintigraphic imaging agent. In yet another preferred embodiment, the agent is a radiotherapeutic agent.

In a second embodiment, the invention provides peptide reagents capable of being radiolabeled for use as scintigraphic imaging agents or radiotherapeutic agents, each comprising a somatostatin analogue that is covalently linked to a radiolabel-binding moiety of formula:

 A-CZ(B)-(C(R′R″))_(n)-X

wherein A is H, HOOC, H₂NOC, (amino acid or peptide)-NHOC, (amino acid or peptide)-OOC or R″″; B is H, SH or -NHR′″, -N(R′″)-(amino acid or peptide) or R″″; X is SH or -NHR′″, -N(R′″)-(amino acid or peptide) or R″″; Z is H or R″″; R′, R″, R′″ and R″″ are independently H or straight or branched chain or cyclic lower alkyl; n is 0, 1 or 2; and: (1) where B is -NHR′″ or -N(R′″)-(amino acid or peptide), X is SH and n is 1 or 2; (2) where X is -NHR′″ or -N(R′″)-(amino acid or peptide), B is SH and n is 1 or 2; (3) where B is H or R″″, A is HOOC, H₂NOC, (amino acid or peptide)-NHOC, (amino acid or peptide)-OOC, X is SH and n is 0 or 1; (4) where A is H or R″″, then where B is SH, X is -NHR′″ or -N(R′″)-(amino acid or peptide) and where X is SH, B is -NHR′″ or -N(R′″)-(amino acid or peptide); (5) where X is H or R″″, A is HOOC, H₂NOC, (amino acid or peptide)-NHOC, (amino acid or peptide)-OOC and B is SH; (6) where Z is methyl, X is methyl, A is HOOC, H₂NOC, (amino acid or peptide)-NHOC, (amino acid or peptide)-OOC and B is SH and n is 0; and (7) where Z is SH and X is SH, n is not 0; and wherein the thiol moiety is in the reduced form.

Preferred embodiments of this radiolabel-binding moiety have a chemical formula that is:

R¹—CO-(amino acid)¹-(amino acid)²-Z

wherein (amino acid)¹ and (amino acid)² are each independently any primary α- or β-amino acid that does not comprise a thiol group, Z is a thiol-containing moiety that is cysteine, homocysteine, isocysteine, penicillamine, 2-mercaptoethylamine or 3-mercaptopropylamine, and R¹ is lower (C¹-C⁴) alkyl, an amino acid or a peptide comprising 2 to 10 amino acids. When Z is cysteine, homocysteine, isocysteine or penicillamine, the carbonyl group of said moiety is covalently linked to a hydroxyl group, a NR³R⁴ group, wherein each of R³ and R⁴ are independently H or lower (C¹-C⁴) alkyl, an amino acid or a peptide comprising 2 to 10 amino acids; or

Y-(amino acid)²-(amino acid)¹-NHR²

wherein Y is a thiol-containing moiety that is cysteine, homocysteine, isocysteine, penicillamine, 2-mercaptoacetate or 3-mercaptopropionate, (amino acid)¹ and (amino acid)² are each independently any primary α- or β-amino acid that does not comprise a thiol group, and R² is H or lower (C¹-C⁴) alkyl, an amino acid or a peptide comprising 2 to 10 amino acids. When Y is cysteine, homocysteine, isocysteine or penicillamine, the amino group of said moiety is covalently linked to -H, an amino acid or a peptide comprising 2 to 10 amino acids; or

wherein n, m and p are each integers that are independently 0 or 1; each R′ is independently H, lower alkyl, C₂-C₄ hydroxyalkyl, or C₂-C₄ alkoxyalkyl, and each R is independently H or R″, where R″ is substituted or unsubstituted lower alkyl or phenyl not comprising a thiol group, and one R or R′ is L, where L is a functionality covalently linked to the somatostatin receptor binding peptide.

In particular embodiments of this aspect of the invention, the radiolabel-binding moiety has a formula that is:

—(amino acid)¹-(amino acid)²-{A-CZ(B)—{C(R¹R²)}_(n)—X},

 —{A-CZ(B)—{C(R¹R²)}_(n)-X}-(amino acid)¹-(amino acid)²,

—(a primary α,ω- or β,ω-diamino acid)-(amino acid)¹-{A-CZ(B)—{C(R¹R²)}_(n)—X},

or

—{A-CZ(B)-{C(R¹R²)}_(n)-X}-(amino acid)¹-(a primary α,ω- or β,ω-diamino acid)

wherein (amino acid)¹ and (amino acid)² are each independently any naturally-ocurring, modified, substituted or altered α- or ,β-amino acid not containing a thiol group; A is H, HOOC, H₂NOC, (amino acid or peptide)-NHOC, (amino acid or peptide)-OOC or R⁴; B is H, SH or —NHR³, —N(R³)-(amino acid or peptide) or R⁴; Z is H or R⁴; X is SH or —NHR³, —N(R³)-(amino acid or peptide) or R⁴; R¹, R², R³ and R⁴ are independently H or straight or branched chain or cyclic lower alkyl; n is an integer that is either 0, 1 or 2; (peptide) is a peptide of 2 to about 10 amino acids; and: (1) where B is —NHR³ or —N(R³)-(amino acid or peptide), X is SH and n is 1 or 2; (2) where X is —NHR³ or —N(R³)-(amino acid or peptide), B is SH and n is 1 or 2; (3) where B is H or R⁴, A is HOOC, H₂NOC, (amino acid or peptide)-NHOC, (amino acid or peptide)-OOC, X is SH and n is 0 or 1; (4) where A is H or R⁴, then where B is SH, X is —NHR³ or —N(R³)-(amino acid or peptide) and where X is SH, B is —NHR³ or —N(R³)-(amino acid or peptide); (5) where X is H or R⁴, A is HOOC, H₂NOC, (amino acid or peptide)-NHOC, (amino acid or peptide)-OOC and B is SH; (6) where Z is methyl, X is methyl, A is HOOC, H₂NOC, (amino acid or peptide)-NHOC, (amino acid or peptide)-OOC and B is SH and n is 0; and (7) where Z is SH and X is SH, n is not 0; and wherein the thiol group is in the reduced form.

Additional preferred embodiments include radiolabel binding moieties having the formula: -Gly-Gly-Cys-, Cys-Gly-Gly-, Gly-Gly-Cys-, -(ε-Lys)-Gly-Cys-, (δ-Orn)-Gly-Cys-, -(γ-Dab)-Gly-Cys-, and -(β-Dap)-Gly-Cys-. (In these formulae, it will be understood that ε-Lys represents a lysine residue in which the ε-amino group, rather than the typical α-amino group, is covalently linked to the carboxyl group of the adjacent amino acid to form a peptide bond; δ-Orn represents an ornithine residue in which the δ-amino group, rather than the typical α-amino group, is covalently linked to the carboxyl group of the adjacent amino acid to form a peptide bond; γ-Dab represents a 2,4-diaminobutyric acid residue in which the γ-amino group is covalently linked to the carboxyl group of the adjacent amino acid to form a peptide bond; and β-Dap represents a 1,3-diaminopropionic acid residue in which the β-amino group is covalently linked to the carboxyl group of the adjacent amino acid to form a peptide bond.)

In another embodiment, the invention provides peptide reagents capable of being radiolabeled with a radioisotope, for radiotherapy or for imaging sites within a mammalian body, each comprising a somatostatin analogue that is covalently linked to a radiolabel-binding moiety of formula:

(for purposes of this invention, radiolabel-binding moieties having this structure will be referred to as picolinic acid (Pic)-based moieties) or

wherein X is H or a protecting group; (amino acid) is any amino acid and the radiolabel-binding moiety is covalently linked to the peptide. For purposes of this invention, radiolabel-binding moieties having this structure will be referred to as picolylamine (Pica)-based moieties. In a preferred embodiment, the amino acid is glycine and X is an acetamidomethyl protecting group.

Yet another embodiment of the invention provides peptide reagents capable of being radiolabeled with a radioisotope, for imaging sites within a mammalian body or for use as a radiotherapeutic agent, each comprising a somatostatin analogue that is covalently linked to a radiolabel-binding moiety that is a bisamino bisthiol radiolabel-binding moiety. The bisamino bisthiol radiolabel-binding moiety in this embodiment of the invention has the formula:

wherein each R can be independently H, CH₃ or C₂H₅; each (pgp)^(s) can be independently a thiol protecting group or H; m, n and p are independently 2 or 3; A is linear or cyclic lower alkyl, aryl, heterocyclyl, combinations or substituted derivatives thereof; and X is peptide; or

wherein each R is independently H, CH₃ or C₂H₅; m, n and p are independently 2 or 3; A is linear or cyclic lower alkyl, aryl, heterocyclyl, combinations or substituted derivatives thereof; V is H or CO-peptide; R′ is H or peptide; provided that when V is H, R′ is peptide and when R′ is H, V is peptide. For purposes of this invention, radiolabel-binding moieties having these structures will be referred to as “BAT” moieties.

This invention provides methods for preparing peptide reagents of the invention by chemical synthesis in vitro. In a preferred embodiment, peptides are synthesized by solid phase peptide synthesis.

This invention provides reagents for preparing a radiolabled somatostatin receptor-binding agent comprising the somatostatin receptor-binding peptides of the invention covalently linked to a radiolabel-binding moiety. In a preferred embodiment, the reagent is radioactively labeled with Tc-99m. In another preferred embodiment, the reagent is radioactively labeled with ¹⁸⁶Re or ¹⁸⁸Re.

The invention also provides complexes of the linear peptide reagents of the invention with a radioisotope, as well as methods for radiolabeling the peptide reagents of the invention. For example, in one embodiment scintigraphic imaging agents provided by the invention comprise Tc-99m labeled complexes formed by reacting the peptide reagents of the invention with Tc-99m in the presence of a reducing agent. Preferred reducing agents include but are not limited to dithionite ion, stannous ion and ferrous ion. Such Tc-99m complexes of the invention are also formed by labeling the peptide reagents of the invention with Tc-99m by ligand exchange of a prereduced Tc-99m complex as provided herein.

The invention also provides kits for preparing radiolabeled linear somatostatin analogue peptides from the peptide reagents of the invention. Kits for radiolabeling the peptide reagents of the invention are comprised of a sealed vial containing a predetermined quantity of a peptide reagent of the invention and a sufficient amount of reducing agent to radiolabel the peptide. In a preferred embodiment, the radiolabeled somatostain analogue is a scintigraphic imaging agent. Also preferred is radiolabeling the peptide reagents of the invention with Tc-99m. Kits for preparing radiotheapeutic agents are also provided, wherein the preferred radioisotopes are rhenium-186 and rhenium-188.

This invention provides methods for using the radiolabeled peptide reagents of the invention diagniostically and therapeutically. In one embodiment of the invention, methods are provided for using scintigraphic imaging agents that are Tc-99m labeled peptide reagents for imaging sites within a mammalian body by obtaining in vivo gamma scintigraphic images. These methods comprise administering an effective diagnostic amount of Tc-99m labeled peptide reagents of the invention and detecting the gamma radiation emitted by the Tc-99m label localized at the site within the mammalian body.

The invention also provides methods for alleviating somatostatin-related diseases in animals, preferably humans, comprising administering a therapeutically effective amount of the radiolabeled somatostatin-binding peptide reagents of the invention to the animal. In preferred embodiments, the reagent is radioactively labeled with ¹⁸⁶Re or ¹⁸⁸Re.

The peptides and peptide reagents of the invention may also be comprised of a polyvalent linking moiety. Polyvalent linking moieties of the invention are comprised of at least 2 identical linker functional groups capable of covalently bonding to somatostatin analogue peptides or Tc-99m binding moieties. Preferred linker functional groups are primary or secondary amines, hydroxyl groups, carboxylic acid groups or thiol-reactive groups. In preferred embodiments, the polyvalent linking moieties are comprised of bis-succinimidylmethylether (BSME), 4-(2,2-dimethylacetyl)benzoic acid (DMBA), N-[2-(N′,N′-bis(2-succinimido-ethyl)aminoethyl)]-N⁶,N⁹-bis(2-methyl-2-mercapto-propyl)-6,9-diazanonanamide (BAT-BS), tris(succinimidylethyl)amine (TSEA), bis-succinimidohexane (BSH), 4-(O-CH₂CO-Gly-Gly-Cys.amide)-2-methylpropiophenone (ETAC), tris(acetamidoethyl)amine, bis-acetamidomethyl ether, bis-acetamidoethyl ether, α,ε-bis-acetyllysine, lysine and 1,8-bis-acetamido-3,6-dioxa-octane, or a derivative thereof.

Specific preferred embodiments of the present invention will become evident from the following more detailed description of certain preferred embodiments and the claims.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides linear peptide reagents for preparing radiolabeled agents for radiodiagnostic and radiotherapeutic uses. The present invention provides linear peptides that are somatostatin analogues and that are not not constrained within a cyclic structure. Such somatostatin analogues thereby possess increased in vivo stability compared with native somatostatin. These linear peptides are themselves therapeutic agents for alleviating diseases and other ailments in animals including humans.

Also provided by the invention are linear peptides that may be radioiodinated or radioastatinated and which are thereby useful in radiotherapeutic and radiodiagnostic applications.

Another embodiment of these linear peptides that is provided by this invention are linear peptide reagents wherein the linear peptides of the invention are covalently linked to a radiolabel-binding moiety. Such linear peptide reagents are capable of being radiolabeled to provide radiodiagnostic or radiotherapeutic agents. One example of a radiodiagnostic application using the radiolabeled agents of the invention is scintigraphic imaging, wherein the location and extent of somatostatin receptor-bearing tumors may be determined.

The linear peptide reagents of the invention can also advantageously be radiolabeled with cytotoxic radioisotopes such as rhenium-186 or rhenium-188 for radiotherapeutic uses. The linear peptide reagents of the invention are also useful in preparing complexes with non-radioactive metals, said complexes being useful therapeutically.

The invention provides a method for using the somatostatin analogues of the invention to alleviate diseases or other ailments in animals, preferably humans. These diseases and ailments include but are not limited to diabetes and diabetes-related retinopathy, cirrhosis of the liver and hepatitis infection, bleeding ulcers and other gastrointestinal bleeding, pancreatitis, central nervous system disorders, endocrine disorders, Alzheimer's disease, acromegaly and other diseases and disorders related to the production of inappropriate levels of growth hormone in vivo, and cancer, particularly those cancers whose growth is dependent or influenced by growth hormone production. Dosages of the somatostatin analogues provided by the invention may be the same as those dosages of native somatostatin routinely used for treatment of the above or other diseases, or less of the compounds of the invention may be administered due to their longer in vivo half-life.

In embodiments of the invention useful as scintigraphic imaging agents, labeling with Tc-99m is an advantage of the present invention because the nuclear and radioactive properties of this isotope make it an ideal scintigraphic imaging agent. This isotope has a single photon energy of 140 keV and a radioactive half-life of about 6 hours, and is readily available from a ⁹⁹Mo-^(99m)Tc generator. Other radionuclides may also be used in the practice of the invention as disclosed herein.

Radiotherapeutic embodiments of the invention, on the other hand, are advantageously labeled with cytotoxic radioisotopes including but not limited to scandium-47, copper-67, gallium-72, yttrium-90, iodine-125, iodine-131, samarium-153, gadolinium-159, dysprosium-165, holmium-166, ytterbium-175, lutetium-177, rhenium-186, rhenium-188, astatine-211 and bismuth-212, most preferably ¹⁸⁶Re or ¹⁸⁸Re. Such embodiments are useful in the treatment of somatostatin-related diseases or other ailments in animals, preferably humans, including but not limited to cancer and other diseases characterized by the growth of malignant or benign tumors capable of binding somatostatin or somatostatin analogues via the expression of somatostatin receptors on the cell surface of cells comprising such tumors.

In the radiolabel-binding moieties and linear peptides covalently linked to such moieties that contain a thiol covalently linked to a thiol protecting groups ((pgp)^(s)) provided by the invention, the thiol-protecting groups may be the same or different and may be but are not limited to:

—CH₂-aryl (aryl is phenyl or alkyl or alkyloxy substituted phenyl);

—CH-(aryl)₂, (aryl is phenyl or alkyl or alkyloxy substituted phenyl);

—C-(aryl)₃, (aryl is phenyl or alkyl or alkyloxy substituted phenyl);

—CH₂-(4-methoxyphenyl);

—CH-(4-pyridyl)(phenyl)₂;

—C(CH₃)₃

-9-phenylfluorenyl;

—CH₂NHCOR (R is unsubstituted or substituted alkyl or aryl);

—CH₂—NHCOOR (R is unsubstituted or substituted alkyl or aryl);

—CONHR (R is unsubstituted or substituted alkyl or aryl);

—CH₂—S—CH₂-phenyl

Preferred protecting groups have the formula —CH₂-NHCOR wherein R is a lower alkyl having 1 and 8 carbon atoms, phenyl or phenyl-substituted with lower alkyl, hydroxyl, lower alkoxy, carboxy, or lower alkoxycarbonyl. The most preferred protecting group is an acetamidomethyl group.

Each somatostatin receptor-binding linear peptide-containing embodiment of the invention is comprised of a sequence of amino acids. The term amino acid as used in this invention is intended to include all L- and D- amino acids, naturally occurring and otherwise. Reagents comprising somatostatin receptor-binding peptides provided by the invention include but are not limited to the following illustrative examples of the peptide embodiments of the invention:

C_(Acm)GC_(Acm)GGGF_(D).Cpa.YW_(D)KTFT. amide

(DTPA).F_(D).Cpa.YW_(D)KTFT(ε-K)GC.amide

maGGGF_(D).Cpa.YW_(D)KTFT.amide

Ac.C_(Acm)GC_(Acm)F_(D).Cpa.YW_(D)KTFT.amide

F_(D).Cpa.YW_(D)KTFTC_(Acm)GC_(Acm).amide

(DTPA).D-Nal.Cpa.YW_(D)KTFT(ε-K)GCKK.amide

AKCGGGF_(D).Cpa.YW_(D)KTFT.amide

(DTPA).D-Nal.Cpa.YW_(D)KTFT(ε-K)GC.amide

F_(D).Cpa.YW_(D)KTFT.GGGC_(Acm)GC_(Acm).amide

(DTPA).Aca.F_(D).Cpa.YW_(D)KTFT(ε-K)GC.amide

(DTPA).(ε-K)GCF_(D).FYW_(D)KTFT.amide

Ac.CGCF_(D).Cpa.YW_(D)KTFT.amide

F_(D).Cpa.YW_(D)KTFTCGC.amide

(DTPA).(D-Nal.CYW_(D)KVCT)₂

Ac.F_(D).FYW_(D)KTFT(ε-K)GC.amide

Ac.F_(D)FYW_(D)KTFTGGG(ε-K)GC.amide

F_(D).Cpa.YW_(D)KTC.Nal.amide

K(BAT).D-Nal.C_(Me)YW_(D)KVC_(Me)T.amide

Ac.F_(D)FYW_(D)KTFGGG(ε-K)KC.amide

Pic.GC_(Acm)GGGF_(D).Cpa.YW_(D)KTFT.amide

(DTPA).D-Nal.CYW_(D)KVCT.amide

(2-ketogulonyl)D-NalFYW_(D)KVCT.amide

F_(D).Cpa.YW_(D)K.Abu.Nal.T(ε-K)GC.amide

(DTPA).K(BAT).D-Nal.C_(Me)YW_(D)KVC_(Me)T.amide

F_(D).Cpa. YW_(D)KTFT(ε-K)GC.amide

(DTPA).F_(D)FYW_(D)KTFT(ε-K)GC.amide

AF_(D)CFW_(D)KTC_(Me)T(CH₂OH)

(DTPA).F_(D)GYW_(D)KTCT(CH₂OH)

(DTPA).Nal.SYW_(D)KVT.K(BAT).amide

(DTPA).Nal.SYW_(D)KVCT.amide

F_(D)FYW_(D)KTFTGGCK.amide

DDD.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide

Ac.DDD.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide

Hca.G.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide

F_(D)FYW_(D)KTFTC_(Acm)GC_(Acm).amide

F_(D)FYW_(D)KTFTGGC.amide

F_(D)FYW_(D)KTFT(ε-K)GC.amide

(Trc.imide)₂K.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCRR.amide

Trc(Trc.imide)K.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCRR.amide

(Trc.imide)Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCR.amide

F_(D).Cpa.YW_(D)KTFT(ε-K)GCR.amide

K_(D)KKK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKDKD.amide

K_(D)KKK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCDD.amide

(Trc)₂K.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide

Hca.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide

(2-ketogulonyl)F_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide

KKKK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCDDDD.amide

Ac.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide

Ac.KKKKK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide

(2-ketogulonyl)F_(D).Cpa.YW_(D)KTFT(ε-K)GC.amide

Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide

DDDD.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKKKK.amide

(DTPA)Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide

(DTPA)Nal_(D)-Cpa.YW_(D)KTFTC_(Acm)GC_(Acm).amide

Ac.KKKKK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GC.amide

KDKD.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKDKD.amide.

As used herein, the following amino acids and amino acid analogues are intended to be represented by the following abbreviations: Ac is an acetyl group; ma is mercaptoacetic acid group; Aca is 6-aminocaproic acid; Hcy is homocysteine; Hhc is homohomocysteine, which is (3-mercaptopropyl)glycine; Pen is penicillamine; Mob is the sulfhydryl protecting group 4-methoxybenzyl; Acm is the sulfhydryl protecting group acetamidomethyl; Aib is aminoisobutyric acid; Nal is 2-naphthylalanine; Ain is 2-amino-indan-2-carboxylic acid; Hly is homolysine; Achxa is 4-amino-cyclohexylalanine; Amf is 4-aminomethylphenylalanine; Aec is S-(2-aminoethyl)cysteine; Apc is S-(3-aminopropyl)cysteine; Aes is O-(2-aminoethyl)serine; Aps is O-(3-aminopropyl)serine; Abu is 2-aminobutyric acid; Nva is norvaline; Aca is 6-aminocaproic acid; F_(D) is D-phenylalanine; W_(D) is D-tryptophan; Y_(D) is D-tyrosine; Cpa is L-(4-chlorophenyl)alanine; Thp is 4-amino-tetrahydrothiopyran-4-carboxylic acid; D-Nal is D-2-naphthylalanine; Dpg is dipropylglycine; Abu is α-aminobutyric acid; Trc is tricarboalkylic acid; Hca is hexacarboxy-cyclohexane; and Nle is norleucine. All naturally-occurring amino acids are abbreviated using standard abbreviations (which can be found in G. Zubay, Biochemistry (2d. ed.), 1988 (MacMillen Publishing: New York) p.33.

For the purposes of this invention, the naturally-occuring amino acids are characterized as lipophilic (alanine, isoleucine, leucine, methionine, phenylalanine, tyrosine, proline, tryptophan and valine, as well as S-alkylated derivatives of cysteine), hydrophilic (asparagine, glutamine, threonine, serine), acidic (glutamic acid and aspartic acid), basic (arginine, histidine and lysine). T(CH ₂ OH) represents a threoninol residue, wherein the carboxyl group of the amino acid is reduced to a primary alcohol, incorporated into the peptide using the procedure of Neugebauer et al. (1990, Peptides: Proceedings of the 11th American Peptide Symposium, pp. 1020-21). ε-K is intended to represent a covalent linkage via the ε-amino group on the sidechain of a lysine residue. δ-Orn represents an ornithine residue in which the δ-amino group, rather than the typical α-amino group, is covalently linked to the carboxyl group of the adjacent amino acid to form a peptide bond. γ-Dab represents a 2,4-diaminobutyric acid residue in which the γ-amino group is covalently linked to the carboxyl group of the adjacent amino acid to form a peptide bond. β-Dap represents a 1,3-diaminopropionic acid residue in which the β-amino group is covalently linked to the carboxyl group of the adjacent amino acid to form a peptide bond. Pic is picolinoyl (pyridine-2-carbonyl); Pica is picolylamine (2-(aminomethyl)pyridine);(BAT)representsN⁶,N⁹-bis(2-mercapto-2-methyl-propyl)-6,9-diazanonanoic acid; K.(BAT) and Lys.(BAT) represent the amino acid lysine, acylated at the ε-amino group on the amino acid sidechain to (BAT); (BAM) is (N¹,N⁴-bis(2-mercapto-2-methylpropyl)-1,4,10-triazadecane; E.(BAM) and Glu.(BAM) represent the amino acid glutamic acid having a γ-amide linkage between the sidechain carboxylic acid group of glutamic acid and a (BAM)-derived primary amino group; (BAT-BM) is N-(2-(N′,N′-bis(2-maleimidoethyl)aminoethyl)-N⁹-(t-butoxycarbonyl)-N⁶,N⁹-bis(2-methyl-2-triphenylmethylthiopropyl)-6,9-diazanonanamide; (BAT-BS) is N-(2-(N′,N′-bis(2-succinimidoethyl)aminoethyl)-N⁶,N⁹-bis(2-mercapto-2-methylpropyl)-6,9-diazanonanamide; (BMME) is bis-maleimidomethylether; (BSME) is bis-succinimidomethylether; and (DTPA) is diethylenetriaminepentaacetic acid.

For the purposes of this invention the term “poly(N-carboxyalkyl)amine” in intended to describe a series of compounds exemplified by nitrilotriacetic acid, iminodiacetic acid, ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA).

For the purposes of this invention the term “polyoxyanion” is intended to encompass sulfates, phosphates, sulfonates, phosphonates and like compounds.

Linear somatostatin analogue peptides of the present invention can be chemically synthesized in vitro. Peptides of the present invention can generally advantageously be prepared on a peptide synthesizer. The peptides of this invention can be synthesized wherein the radiolabel-binding moiety is covalently linked to the peptide during chemical synthesis in vitro, using techniques well known to those with skill in the art. Such peptides covalently-linked to the radiolabel-binding moiety during synthesis are advantageous because specific sites of covalent linkage can be determined.

Radiolabel binding moieties of the invention may be introduced into the target linear somatostatin analogue peptides during peptide synthesis. For embodiments comprising picolinic acid ((Pic-); e.g., Pic-Gly-Cys(protecting group)-), the radiolabel-binding moiety can be synthesized as the last (i.e., amino-terminal) residue in the synthesis. In addition, the picolinic acid-containing radiolabel-binding moiety may be covalently linked to the ε-amino group of lysine to give, for example, αN(Fmoc)-Lys-εN(Pic-Gly-Cys(protecting group)), which may be incorporated at any appropriate position in the peptide chain. This sequence is particularly advantageous as it affords an easy mode of incorporation into the target somatostatin analogue peptide.

Similarly, the picolylamine (Pica)-containing radiolabel-binding moiety (-Cys(protecting group)-Gly-Pica) can be prepared during peptide synthesis by including the sequence (-Cys(protecting group)-Gly-) at the carboxyl terminus of the peptide chain. Following cleavage of the peptide from the resin the carboxyl terminus of the peptide is activated and coupled to picolylamine. This synthetic route requires that reactive side-chain functionalities remain masked (protected) and do not react during the conjugation of the picolylamine.

This invention also provides small linear synthetic peptides that are somatostatin analogues and incorporate bisamine bisthiol (BAT) chelators that may be labeled with Tc-99m.

This invention provides for the incorporation of these chelators into virtually any position in the peptide, via covalently linkage to any appropriate functional group of the peptide, except that the chelating moieties of the invention are not covalently linked to functional groups comprising the amino acid side chains of the amino acids B¹, B², B³ or B⁴.

In forming a complex of radioactive technetium with the reagents of this invention, the technetium complex, preferably a salt of Tc-99m pertechnetate, is reacted with the reagent in the presence of a reducing agent. Preferred reducing agents are dithionite, stannous and ferrous ions; the most preferred reducing agent is stannous chloride. Means for preparing such complexes are conveniently provided in a kit form comprising a sealed vial containing a predetermined quantity of a reagent of the invention to be labeled and a sufficient amount of reducing agent to label the reagent with Tc-99m. Alternatively, the complex may be formed by reacting a reagent of this invention with a pre-formed labile complex of technetium and another compound known as a transfer ligand. This process is known as ligand exchange and is well known to those skilled in the art. The labile complex may be formed using such transfer ligands as tartrate, citrate, gluconate or mannitol, for example. Among the Tc-99m pertechnetate salts useful with the present invention are included the alkali metal salts such as the sodium salt, or ammonium salts or lower alkyl ammonium salts.

In a preferred embodiment of the invention, a kit for preparing technetium-labeled peptides is provided. An appropriate amount of the peptide reagent is introduced into a vial containing a reducing agent, such as stannous chloride, in an amount sufficient to label the peptide with Tc-99m. An appropriate amount of a transfer ligand as described (such as tartrate, citrate, gluconate or mannitol, for example) can also be included. The kit may also contain conventional pharmaceutical adjunct materials such as, for example, pharmaceutically acceptable salts to adjust the osmotic pressure, buffers, preservatives and the like. The components of the kit may be in liquid, frozen or dry form. In a preferred embodiment, kit components are provided in lyophilized form.

Technetium-99m labeled imaging reagents according to the present invention may be prepared by the addition of an appropriate amount of Tc-99m or Tc-99m complex into the vials and reaction under conditions described in Example 2 hereinbelow.

Radioactively-labeled scintigraphic imaging agents provided by the present invention are provided having a suitable amount of radioactivity. In forming Tc-99m radioactive complexes, it is generally preferred to form radioactive complexes in solutions containing radioactivity at concentrations of from about 0.01 millicurie (mCi) to 100 mCi per mL.

The imaging reagents provided by the present invention can be used for visualizing organs such as the kidney for diagnosing disorders in these organs, and tumors, in particular gastrointestinal tumors, myelomas, small cell lung carcinoma and other APUDomas, endocrine tumors such as medullary thyroid carcinomas and pituitary tumors, brain tumors such as meningiomas and astrocytomas, and tumors of the prostate, breast, colon, and ovaries can also be imaged. In accordance with this invention, the Tc-99m labeled peptide reagents are administered in a single unit injectable dose. The Tc-99m labeled peptide reagents provided by the invention may be administered intravenously in any conventional medium for intravenous injection such as an aqueous saline medium, or in blood plasma medium. Generally, the unit dose to be administered has a radioactivity of about 0.01 mCi to about 100 mCi, preferably 1 mCi to 20 mCi. The solution to be injected at unit dosage is from about 0.01 mL to about 10 mL. After intravenous administration, imaging in vivo can take place in a matter of a few minutes. However, imaging can take place, if desired, in hours or even longer, after the radiolabeled peptide is injected into a patient. In most instances, a sufficient amount of the administered dose will accumulate in the area to be imaged within about 0.1 of an hour to permit the taking of scintiphotos. Any conventional method of scintigraphic imaging for diagnostic purposes can be utilized in accordance with this invention.

The somatostatin receptor-binding linear peptides and non-radioactive metal complexes of the linear peptide reagents of the invention may be used clinically to promote regression of certain types of tumors, particularly those that express somatostatin receptors. The linear somatostatin analogue peptides of the invention can also be used to reduce the hormonal hypersecretion that often accompanies certain cancers, such as the APUDomas. Peptides of the invention used as therapeutic agents may be administered by any appropriate route, including intravenous, intramuscular or by mouth, and in any acceptable pharmaceutical carrier, in doses ranging from about 0.1 to about 49 mg/kgbody weight/day.

This invention also provides peptides radiolabled with a cytotoxic radioisotope such as rhenium-186 or rhenium-188 that may be used for radiotherapy of certain tumors as described above. For this purpose, an amount of radioactive isotope from about 10 mCi to about 200 mCi may be administered via any suitable clinical route, preferably by intravenous injection.

The methods for making and labeling these compounds are more fully illustrated in the following Examples. These Examples illustrate certain aspects of the above-described method and advantageous results, and are shown by way of illustration and not limitation.

EXAMPLE 1 Solid Phase Peptide Synthesis

Solid phase peptide synthesis (SPPS) was carried out on a 0.25 millimole (mmole) scale using an Applied Biosystems Model 431A Peptide Synthesizer and using 9-fluorenylmethyloxycarbonyl (Fmoc) amino-terminus protection, coupling with dicyclohexylcarbodiimide/hydroxybenzotriazoleor2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate/hydroxybenzotriazole (HBTU/HOBT), and usingp-hydroxymethylphenoxy-methylpolystyrene (HMP) resin for carboxyl-terminus acids or Rink amide resin for carboxyl-terminus amides.

Where appropriate, the following amino acid derivatives were synthesized. Homocysteine was prepared by alkaline hydrolysis of L-homocysteine lactone. Threoninol residues, wherein the carboxyl group of the amino acid is reduced to a primary alcohol, can be introduced into the peptides of the invention where appropriate using the procedure of Neugebauer et al. (1990, Peptides: Proceedings of the 11th American Peptide Symposium, pp. 1020-21). Fmoc.Hcy(Trt) and Fmoc.Pen(Trt) were prepared from the appropriate amino acids by tritylation with triphenylmethanol in TFA, followed by Fmoc derivitization as described by Atherton et al. (1989, Solid Phase Peptide Synthesis, IRL Press: Oxford). Fmoc.homohomocysteine(Trt) was prepared by reducing N,N-bis-Boc-glutamic acid-α-methyl ester with borane-THF, followed by mesylation and reaction with trityl-mercaptide, followed by removal of the Boc groups with BF₃OEt in acetic acid, and then Fmoc derivitization as described above. PhCH₂CHBrCOOH was prepared by treating phenylalanine (in a solution of water and TFA/saturated with NaBr) with sodium nitrite, followed by distillation to recover the pure product.

Where appropriate, 2-chloroacetyl, 2-bromoacetyl and 2-bromo-3-phenylpropionyl groups were introduced either by using the appropriate 2-halo acid as the last residue coupled during SPPS, or by treating the N-terminus free amino acid peptide bound to the resin with either 2-halo acid/diisopropylcarbodiimide/N-hydroxysuccinimide/NMP or 2-halo acid anhydride/diisopropylethylamine/NMP.

Where appropriate, HPLC-purified 2-haloacylated peptides were cyclized by stirring an 0.1-1.0 mg/mL solution in phosphate or bicarbonate buffer or dilute ammonium hydroxide (pH 8.0), optionally containing 0.5-1.0 mM EDTA, or acetonitrile or THF for 1-48 h followed optionally by acidification with acetic acid, lyophilization and HPLC purification.

Where appropriate, (BAM) (N¹,N¹-bis(2-mercapto-2-methylpropyl)-1,4,10-triazadecane) was conjugated to the peptide by first activating the peptide carboxylate with a mixture of diisopropylcarbodiimide/N-hydroxysuccinimide or HBTU/HOBt in DMF, NMP or methylene chloride, followed by coupling in the presence of diisopropylethylamine. After coupling, the conjugates were deprotected as described above.

Where appropriate, (BAT) (N⁶,N⁹-bis(2-mercapto-2-methylpropyl)-6,9-diazanonanoic acid) was incorporated into peptide as (Nα(Fmoc)-Nε(N-Boc)-S,S′-bistrityl-BAT)lysine (prepared from Nα(Fmoc)-lysine and Nε(N-Boc)-S,S′-bistrityl-BAT incorporated by reference) during peptide synthesis and then deprotected after cleavage of the completed peptide from the synthetic resin.

Where appropriate, BSME adducts were prepared by reacting single thiol-containing peptides (5 to 50 mg/mL in DMF buffered to pH 7 with N-methylmorpholine or N-ethyl-morpholine, or 50 mM sodium phosphate buffer, pH 7-8, optionally containing 0.5 mM EDTA or DMF or THF or acetonitrile) with 0.5 molar equivalents of BMME (bis-maleimidomethylether) pre-dissolved in acetonitrile at room temperature for approximately 1-18 hours. The solution was concentrated and the product was purified by HPLC.

Where appropriate, TSEA adducts were prepared by reacting single thiol-containing peptide (at concentrations of 10 to 100 mg/mL peptide in DMF buffered to pH 7 with N-methylmorpholine or N-ethylmorpholine, or 5 to 50 mg/mL peptide in 50 mM sodium phosphate, pH 7-8, optionally containing 0.5 mM EDTA or DMF or THF or acetonitrile) with 0.33 molar equivalents of TMEA (tris(2-maleimidoethyl)amine) pre-dissolved in acetonitrile or DMF, with or without 1 molar equivalent of triethanolamine, at room temperature for approximately 1-18 h. Such reaction mixtures containing adducts were concentrated and the adducts were then purified using HPLC.

Where appropriate, BAT-BS (N-(2-(N′,N′-bis(2-succinimidoethyl) aminoethyl))-N⁶,N⁹-bis(2-methyl-2-mercaptopropyl)-6,9-diazanonanamide) adducts were prepared by reacting single thiol-containing peptide (at concentrations of 2 to 50 mg/mL peptide in DMF buffered to pH 7 with N-methyl-morpholine or N-ethyl-morpholine, or in 50 mM sodium phosphate (pH 7-8), optionally containing 0.5 mM EDTA or DMF or THF or acetonitrile) with 0.5 molar equivalents of BAT-BM (N-(2-(N′,N′-bis(2-maleimidoethyl)aminoethyl))-N⁹-(t-butoxycarbonyl)-N⁶,N⁹-bis(2-methyl-2-triphenylmethylthiopropyl)-6,9-diazanonanamide) pre-dissolved in acetonitrile or THF, at room temperature for approximately 1-18 h. The solution was then evaporated to dryness and (BAT-BS)-peptide conjugates deprotected by treatment with 10 mL TFA and 0.2 mL triethylsilane for 1h. The solution was concentrated, the product adducts precipitated with ether, and then purified by HPLC.

Where appropriate, the (DTPA) moiety can be introduced using the method of Bakker et al. (1991, Life Sci. 49: 1583-1591, hereby incorporated by reference).

Resin-bound products were routinely cleaved using a solution of trifluoroacetic acid or trifluoroacetic acid and methylene chloride, optionally containing water, thioanisole, ethanedithiol, and triethylsilane, prepared in ratios of 100:5:5:2.5:2 for 0.5-3 h at room temperature. Crude peptides were purified by preparative high pressure liquid chromatography (HPLC) using a Waters Delta Pak C18 column and gradient elution using 0.1% trifluoroacetic acid (TFA) in water modified with acetonitrile. Acetonitrile was evaporated from the eluted fractions which were then lyophilized. The identity of each product was confirmed by fast atom bombardment mass spectroscopy (FABMS) or by electrospray mass spectroscopy (ESMS).

Somatostatin analogues synthesized as provided herein, as well as the products of such synthesis identified by FABMS, are shown in Table I below.

EXAMPLE 2 A General Method for Radiolabeling

0.1 mg of a peptide prepared as in Example 2 was dissolved in 0.1 mL of water or 50/50 ethanol/water or phosphate-buffered saline or 50 mM potassium phosphate buffer (pH=5, 6 or 7.4). Tc-99m gluceptate was prepared by reconstituting a Glucoscan vial (E.I. DuPont de Nemours, Inc.) with 1.0 mL of Tc-99m sodium pertechnetate containing up to 200 mCi and allowed to stand for 15 minutes at room temperature. 25 μl of Tc-99m gluceptate was then added to the peptide and the reaction allowed to proceed at room temperature or at 100° C. for 15-30 min and then filtered through a 0.2 μm filter.

The Tc-99m labeled peptide purity was determined by HPLC using the following conditions: a Waters Delta Pak RP-18, 5μ, 4.6mm×220mm analytical column was loaded with each radiolabeled peptide, and the peptides eluted at a solvent flow rate equal to 1 mL/min. Gradient elution was performed beginning with 100% solvent A (0.1% CF₃COOH/H₂O) and ending with 1005 solvent B₉₀ (0.1% CF₃COOH/90% CH₃CN/H₂O) over the course of 10-20 min.

Radioactive components were detected using an in-line radiometric detector linked to an integrating recorder. Tc-99m gluceptate and Tc-99m sodium pertechnetate elute between 1 and 4 minutes under these conditions, whereas the Tc-99m labeled peptides eluted after a much greater amount of time, as illustrated in Table I below.

TABLE I Peptide MH⁺ RCY (%) R_(t) (min) C_(Acm)GC_(Acm)GGGF_(D).Cpa.YW_(D)KTFT.amide 1749 97⁶ 15.7² (DTPA).F_(D).Cpa.YW_(D)KTFT(ε-K)GC.amide 1837 97⁷ 15.5² ma.GGGF_(D).Cpa.YW_(D)KTFT.amide 1417 98⁶ 12.2³ Ac.C_(Acm)GC_(Acm)F_(D).Cpa.YW_(D)KTFT.amide 1619 75⁶ 17.1,17.5² F_(D).Cpa.YW_(D)KTFTC_(Acm)GC_(Acm).amide 1577 93⁵ 12.1³ (DTPA).D-Nal.Cpa.YW_(D)KTFT(ε-K)GCKK.amide 2143 N.D. N.D. AKCGGGF_(D).Cpa.YW_(D)KTFT.amide 1612 98⁷ 15-16² (DTPA).D-Nal.Cpa.YW_(D)KTFT(ε-K)GC.amide 1887 97⁷ 16.2² F_(D).Cpa.YW_(D)KTFT.GGGC_(Acm)GC_(Acm).amide 1749 76⁵ 17.7,18.0¹ (DTPA).Aca.F_(D).Cpa.YW_(D)KTFT(ε-K)GC.amide 1950 97³ 11.5³ (DTPA).(ε-K)GCF_(D).FYW_(D)KTFT.amide 1802 97³ 11.5³ AC.CGCF_(D).Cpa.YW_(D)KTFT.amide 1477 98⁸ 18.1² F_(D).Cpa.YW_(D)KTFTCGC.amide 1435 99⁸ 16.8,17.0² (DTPA).(D-Nal.CYW_(D)KVCT)₂ 2554 97⁸ 11.8-12.4³ Ac.F_(D).FYW_(D)KTFT(ε-K)GC.amide 1469 96³ 12.1,12.6³ Ac.F_(D)FYW_(D)KTFTGGG(ε-K)GC.amide 1640 98⁸ 11.9,12.4³ F_(D).Cpa.YW_(D)KTC.Nal.amide 1224 88⁸ 18.6,20.4² KDKD.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKDKD.amide 2484 99⁶ 14.8 Ac.KKKKK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide 2450 98⁶ 14.2 Ac.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide 1810 96⁶ 16.8 KKKK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCDDDD.amide 2485 98⁶ 14.6 (2-ketogulonyl)F_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide 1944 99⁶ 16.0 Hca.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide 2097 98⁶ 15.8 (Trc)₂K.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide 2212 98⁶ 15.7 KDKKK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCDD.amide 2253 99⁶ 14.7 KDKKK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKDKD.amide 2485 79⁶ 14.1 F_(D).Cpa.YW_(D)KTFT(ε-K)GCR.amide 1617 99⁶ 15.4 (Trc.imide)Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCR.amide 1808 99⁶ 17.3 Trc(Trc.imide)K.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCRR.amide 2250 99⁶ 16.2 (Trc.imide)₂K.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCRR.amide 2232 99⁶ 16.6 F_(D)FYW_(D)KTFT(ε-K)GC.amide 1427 99⁶ 15.1 F_(D)FYW_(D)KTFTGGC.amide 1356 99⁶ 15.2-16.2 F_(D)FYW_(D)KTFTGGCK.amide 1484 99⁶ 14.9,15.6³ DDD.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide 1998 99⁵ 15.6³ Ac.DDD.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide 2040 100⁵  16.0³ DDDD.Nal_(D).Cpa.YW_(D)KTFT.(ε-K).GCKKKK.amide 2484 98⁶ 15.1 (DTPA).Nal_(D).Cpa.YW_(D)KTFT.(ε-K).GCKK.amide 2192 95⁶ 15.8³ (DTPA).Nal_(D).Cpa.YW_(D)KTFTC_(Acm)GC_(Acm).amide 2003 93⁷ 16.4³ AC.KKKKK.Nal_(D).Cpa.YW_(D)KTFT.(ε-K).GC.amide 2192 94² 14.9³ Hca.G.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide 2136 93⁵ 16.0³ (DTPA).F_(D)FYW_(D)KTFT(ε-K)GC.amide 1801 97⁵ 11.3³ F_(D).Cpa.YW_(D)KTFT(ε-K)GC.amide 1461 98⁷ 15.8² (DTPA).K.(BAT).D-Nal.C_(Me)YW_(D)KVC_(Me)T.amide 1949 96⁸ 12.3³ F_(D).Cpa.YW_(D)K.Abu.Nal.T(ε-K)GC.amide 1495 95⁷ 16.5² (2-ketogulonyl)-D-NalFYW_(D)KVCT.amide 1318 98³ 12.4,13.0³ (DTPA).D-Nal.CYW_(D)KVCT.amide 1473 97⁸ 11.0³ Pic.GC_(Acm)GGGF_(D).Cpa.YW_(D)KTFT.amide 1681 98⁸ 13.8-16.8¹ Ac.F_(D)FYW_(D)KTFGGG(ε-K)KC.amide 1710 98⁸ 15.9² K.(BAT).D-Nal.C_(Me)YW_(D)KVC_(Me)T.amide 1573 97⁸ 12.5³ AF_(D)CFW_(D)KTC_(Me)T(CH₂OH) 1106 99² 11.3-11.9³ (DTPA).F_(D)GYW_(D)KTCT(CH₂OH) N.D. 96³ 10.6¹ (DTPA).Nal.SYW_(D)KVTK.(BAT).amide 1801 96³ 12.0³ (DTPA).Nal.SYW_(D)KVCT.amide 1457 95⁸ 11.6³ Nal_(D).Cpa.YW_(D)KTFT.(ε-K).GCKK.amide 1767 98⁶ 15.8 (2-ketoguonyl).F_(D).Cpa.YW_(D)KTFT.(ε-K).GC.amide 1636 99² 15.8 F_(D)FYW_(D)KTFTC_(Acm)GC_(Acm).amide 1544 N.D. N.D. *The following labeling conditions were used with the appropriate peptides: 1. The peptide is dissolved in 50 mM potassium phosphate buffer (pH 7.4) and labeled at room temperature. 2. The peptide is dissolved in water and labeled at room temperature. 3. The peptide is dissolved in water and labeled at 100° C. 4. The peptide is dissolved in 50% ethanol/water and labeled at 100° C. 5. The peptide is dissolved in 10% hydroxypropylcyclodextrin and labeled at room temperature. 6. The peptide is dissolved in 50% ethanol/water and labeled at room temperature. 7. The peptide is dissolved in water adjusted to pH 9 and labeled at 100° C. 8. The peptide is dissolved in water adjusted to pH 6.5 and labeled at 100° C. ** HPLC methods: general: solvent A = 0.1% CF3COOH/H₂O solvent B = 0.1% CF₃COOH/90% CH₃CN/H₂O solvent flow rate = 1 mL/min Columns: a. Vydak column = Vydak 218TP54 RP-18, 5μ × 220 mm × 4.6 mm anaiytical column with guard column b. Waters column = Waters Delta-Pak C18, 5 μm, 39 × 150 mm Method 1: Vydak column 100% A to 100% B₉₀ in 10 min Method 2: Waters column 100% A to 100% B₉₀ in 20 min Method 3: Waters column 100% A to 100% B₉₀ in 10 min

Single-letter abbreviations for amino acids can be found in G. Zubay, Biochemistry (2d. ed.), 1988 (MacMillen Publishing: New York) p.33; Ac=acetyl; Acm=acetamidomethyl; ma=mercaptoacetic acid; Aca=6-aminocaproic acid; Hly=homolysine; Apc=L-(S-(3-aminopropyl)cysteine; F_(D)=D-phenylalanine; W_(D)=D-tryptophan; Y_(D)=D-tyrosine; Cpa=L-(4-chlorophenyl)alanine; D-Nal=D-2-naphthylalanine; Nle=norleucine; Hcy=homocysteine; Hhc=homohomocysteine; Pen=penicillamine; Aib=aminoisobutyric acid; Nal=2-naphthylalanine; D-Nal=D-2-naphthylalanine; Ain=2-aminoindane-2-carboxylic acid; Achxa=4-amino-cyclohexylalanine; Amf=4-aminomethyl-phenylalanine; Aec=S-(2-aminoethyl)cysteine; Apc=S-(3-aminopropyl)cysteine; Aes=O-(2-aminoethyl)serine; Aps=O-(3-aminopropyl)serine; Abu=2-aminobutyric acid; Trc=tricarboallylic acid; Hca=hexacarboxycyclohexane; Nva=norvaline; T(CH ₂ OH) OH)=threoninol (on which the carboxylic acid moiety has been reduced to a primary alcohol); ε-K=a lysine residue in a peptide in which the peptide bond involves the ε-amino group on the lysine sidechain rather than the α-amino group; δ-Orn=an ornithine residue in which the δ-amino group, rather than the typical α-amino group, is covalently linked to the carboxyl group of the adjacent amino acid to form a peptide bond; γ-Dab=a 2,4-diaminobutyric acid residue in which the γ-amino group is covalently linked to the carboxyl group of the adjacent amino acid to form a peptide bond; β-Dap=a 1,3-diaminopropionic acid residue in which the β-amino group is covalently linked to the carboxyl group of the adjacent amino acid to form a peptide bond; Pic=picolinoyl (pyridine-2-carbonyl); Pica=picolylamine (2-(aminomethyl)pyridine); BAT=N⁶,N⁹-bis(2-mercapto-2-methylpropyl)-6,9-diazanonanoic acid; BAT acid (protected)=N⁹-(t-butoxycarbonyl)-N⁶,N⁹-bis(2-methyl-2-triphenylmethylthiopropyl)-6,9-diazanonanoic acid; BAM=N¹,N⁴-bis(2-mercapto-2-methylpropyl)-1,4,10-triazadecane; BAM (protected)=N¹-(t-butoxycarbonyl)-N¹,N⁴-bis(2-methyl-2-triphenylmethylthiopropyl)-1,4,10-triazadecane; (BAT-BM)=N-(2-(N′,N′-bis(2-maleimidoethyl)aminoethyl)-N⁹-(t-butoxycarbonyl)-N⁶,N⁹-bis(2-methyl-2-triphenylmethylthiopropyl)-6,9-diazanonanamide; (BAT-BS)=N-(2-(N′,N′-bis(2 - succinimidoethyl)aminoethyl)-N⁶,N⁹-bis(2-mercapto-2-methylpropyl)-6,9-diazanonanamiide; (BMME)=bis-maleimidomethylether; (BSME)=bis-succinimidomethylether; (DTPA)=diethylenetriaminepentaacetic acid. RCY(%)=radiochemical yield (determined by HPLC)

Non-radioactive rhenium complexes were prepared by co-dissolving each of the peptide reagents of the invention with about one molar equivalent of tetrabutylammonium oxotetra-bromorhenate (+5), prepared as described by Cotton et al. (1966, Inorg. Chem. 5: 9-16) in dimnethylformamide or acetonitrile/water and stirred for 0.5-5 days. The rhenium complexes were isolated by reverse phase HPLC as described above for Tc-99m labeled peptides and were characterized by FABMS or ESMS.

Radioactive rhenium complexes, using for example Re-186 or Re-188, are prepared from the appropriate perrhenate salts using the same protocol as for Tc-99m labeling, or by adding a reducing agent to a solution of the peptide and perrhenate, or optionally using a ligand transfer agent such as citrate and incubating the reaction at a temperature between room temperature and 100° C. for between 5 and 60 min.

EXAMPLE 3 Inhibition of Binding of (¹²⁵I-Tyr¹¹)somatostatin-14 to AR42J Rat Pancreatic Tumor Cell Membranes

The ability of various somatostatin analogues of the invention to bind to somatostatin receptors in vitro was demonstrated by assaying the ability of such analogues to inhibit binding of a radiolabeled somatostatin analogue to somatostatin receptor-containing cell membranes. The rat pancreatic tumor cell line AR42J which expresses the somatostatin receptor was cultured in Dulbecco's minimal essential media (DMEM) supplemented with 10% fetal bovine serum (FBS) and 8 mM glutamine in a humdified 5% CO₂ atmosphere at 37° C. in T-flasks. Harvested cells were homogenized in cold 50 mM Tris-HCl buffer (pH 7.4) and the homogenate then centrifuged at 39,000g for 10 min at 4° C. Pellets were washed once with buffer and then resuspended in an ice-cold solution of 10mM Tris-HCl (pH 7.4). Equal aliquots of this cell membrane preparation were incubated with (¹²⁵I-Tyr¹¹)somatostatin-14 (at a final concentration of 0.5nM and 750,000cpm/mL, at a specific activity of 2000Ci/mmol, Amersham, Arlington Heights, Ill.) and peptide at a final concentration of from 10⁻¹¹M to 10⁻⁶M in a solution of 50 mM HEPES (pH 7.4) containing 1% bovine serum albumin (BSA), 5 mM MgCl₂, Trasylol (200,000 International Units), bacitracin (0.02 mg/mL) and phenylmethylsulfonylfluoride (0.02 mg/mL) for 25 min at 30° C. Using a filtration manifold, this mixture was filtered through a polyethyleneimine-washed GC/F filter (Whatman, Maidstone, England), and the residue remaining on the filter washed thrice with 5 mL cold HEPES buffer. The filter and a sample of the filter washings were then counted in a gamma counter. To assess non-specific binding, the assay was performed in the presence of unlabeled somatostatin-14 at 200 nM. Data analysis including Hill plots of the data provided inhibition constants (see Bylund & Yamamura, “Methods of receptor binding”, in Methods in Neurotransmitter Receptor Analysis, Yamamura et al., eds., Raven Press: New York, 1990).

These results are presented in the following Table. The data show that the peptides of the instant invention have a high affinity of binding for somatostatin receptors.

It should be understood that the foregoing disclosure emphasizes certain specific embodiments of the invention and that all modifications or alternatives equivalent thereto are within the spirit and scope of the invention as set forth in the appended claims.

TABLE II Peptide K_(i) (nM) C_(Acm)GC_(Acm)GGGF_(D).CPa.YW_(D)KTFT.amide <0.01 (DTPA)F_(D).Cpa.YW_(D)KTFT(ε-K)GC.amide 0.24 maGGGF_(D).Cpa.YW_(D)KTFT.amide 0.25 cyclo(N-Me)FYW_(D)KV.Hcy(CH₂CO.GGCKK.amide) 0.26 Ac.C_(Acm)GC_(Acm)F_(D).Cpa.YW_(D)KTFT.amide 0.73 F_(D).Cpa.YW_(D)KTFTC_(Acm)GC_(Acm).amide 0.85 (DTPA)Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide 1.3 AKCGGGF_(D).Cpa.YW_(D)KTFT.amide 1.4 (DTPA)Nal_(D).Cpa.YW_(D)KTFT(ε-K)GC.amide 2.0 (DTPA)Nal_(D).Cpa.YW_(D)KT.Nal.T(ε-K)GCKK.amide 2.0 F_(D).Cpa.YW_(D)KTFTGGGC_(Acm)GC_(Acm).amide 2.4 (DTPA).Aca._(D).Cpa.YW_(D)KTFT(ε-K)GC.amide 2.6 KDKD.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKDKD.amide 2.6 (2-ketogulonyl)F_(D).Cpa.YW_(D)KTFT(ε-K)GC.amide 2.7 (DTPA).(ε-K)GCF_(D).Cpa.YW_(D)KTFT.amide 3.3 AC.CGCF_(D).Cpa.YW_(D)KTFT.amide 4.4 F_(D).Cpa.YW_(D)KTFTCGC.amide 4.8 K_(D)KKK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKDKD.amide 4.9 Nal_(D).Cpa YW_(D)KTFT(ε-K)GCKK.amide 5.6 Ac.KKKKK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GC.amide 6.5 KKKK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCDDDD.amide 6.9 (DTPA)(Nal_(D).CYW_(D)KVCT)₂ 7.2 Ac.KKKKK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide 7.7 Ac.F_(D)FYW_(D)KTFT(ε-K)GC.amide 7.9 Ac.F_(D)FYW_(D)KTFTGGG(ε-K)GC.amide 8.2 F_(D).Cpa.YW_(D)KTC.Nal.amide 8.2 K(BAT).Nal_(D).C_(Me)YW_(D)KVC_(Me)T.amide 9.9 (Re═O)-Compexed Peptides MH⁺ K_(i) (nM) F_(D).Cpa.YW_(D)KTC(ε-K)GCKK.amide 1917 0.13 Ac.D_(D)F_(D).Cpa.YW_(D)KTC(ε-K)GCKK.amide 2074 0.20 (DTPA)Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide 2343 0.33 F_(D).Cpa.YW_(D)KTC(ε-K)GC.amide N.D. 0.36 F_(D).Cpa.YW_(D)KTC(ε-K)CGC.amide 1635 0.37 F_(D)FYW_(D)KTFTGGC.amide 1683 0.37 F_(D)FYW_(D)KTFTGGCK.amide 2032 0.38 C_(Acm)GC_(Acm)GGGF_(D).Cpa.YW_(D)KTFT.amide 1807 0.43 D_(D)DF_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide 2147 0.50 F_(D)FYW_(D)KTFTC_(Acm)GC_(Acm).amide 1601 0.58 Ac.KKKKK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide 2649 0.63 (DTPA)Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide 2393 0.67 AKCGGGF_(D)FYW_(D)KTFT.amide 1812 0.76 KKKK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCDDDD.amide 2683 0.83 maGGGF_(D).Cpa.YW_(D)KTFT.amide 1618 0.97 F_(D).Cpa.YW_(D)KTFT(ε-K)GCR.amide 1817 1.3 Ac.D_(D)DF_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide 2188 1.4 DDD.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide 2197 1.4 KDKD.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKDKD.amide 2083 1.4 Ac.F_(D)FYW_(D)KTFT(ε-K)GC.amide 1688 1.5 K_(D)KK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCDDD.amide 2440 1.5 K_(D)KK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCDD.amide 2453 1.6 Ac.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide 2008 1.9 Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide 1967 2.2 AKCGGGF_(D)FYW_(D)KTFT.amide 1812 2.9 (DTPA)Nal_(D).Cpa.YW_(D)KTFTC_(Acm)GC_(Acm).amide 2061 3.1 F_(D).Cpa.YW_(D)K.Abu.Nal.T(ε-K)GC.amide 1695 3.3 (2-ketogulonyl)F_(D).Cpa.YW_(D)KTFT(ε-K)GC.amide 1837 3.7 KDKKK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKDKD.amide 2684 3.8 Ac.CGCF_(D).Cpa.YW_(D)KTFT.amide 1677 4.1 F_(D)FYW_(D)KTFT(ε-K)GC.amide 1637 4.3 Ac.KKKKK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GC.amide 2394 4.4 (Trc.imide)₂K.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCRR.amide 2432 4.9 (2-ketogulonyl)F_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide 2143 5.2 Ac.DDD.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide 2239 6.0 Ac.F_(D)FYW_(D)KTFTGGG(ε-K)KC.amide 1911 6.1 (DTPA).F_(D).Cpa.YW_(D)KTFT(ε-K)GC.amide 2036 7.9 Hca.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK, amide 2298 8.0 K_(D)KKKF_(D).Cpa.YW_(D)KTF.Nal.(ε-K)GCDDDD.amide 2730 8.1 Ac.F_(D)FYW_(D)KTFTGGG(ε-K)GC.amide 1840 8.1 (DTPA).Aca.F_(D).Cpa.YW_(D)KTFT(ε-K)GC.amide 2149 8.2 DDDD.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKKKK.amide 2674 9.8 (DTPA).Nal_(D).Cpa.YW_(D)KTFT(ε-K)GC.amide 2085 11

23 14 amino acids amino acid circular peptide unknown Modified-site 3..14 /label= Disulfide bond /note= “A disulfide bond exists between the two sulfur atoms of the cysteine residues; 1 Ala Gly Cys Lys Asn Phe Phe Trp Lys Thr Phe Thr Ser Cys 1 5 10 14 amino acids amino acid linear peptide unknown Modified-site 1..3 /label= Protecting group /note= ”Each of the side chain sulfur atoms of the two cysteine residues is protected by an acetamidomethyl group; Modified-site 7..10 /label= Variant residues /note= “The Phe is in the D conformation; Xaa is L-4-chlorophenylalanine; the Trp is in the D conformation; 2 Cys Gly Cys Gly Gly Gly Phe Xaa Tyr Trp Lys Thr Phe Thr 1 5 10 11 amino acids amino acid linear peptide unknown Modified-site 1..2 /label= N-terminus group /note= ”The amino terminus is acetylated by a mercaptoacetic acid group; Modified-site 4..7 /label= Variant residues /note= “The Phe is in the D conformation; Xaa is L-4-chlorophenylalanine; the Trp is in the D conformation; 3 Gly Gly Gly Phe Xaa Tyr Trp Lys Thr Phe Thr 1 5 10 11 amino acids amino acid linear peptide unknown Modified-site 1..3 /label= Protecting group /note= ”Each of the side chain sulfur atoms of the two cysteine residues is protected by an acetamidomethyl group; the N terminus is acetylated; Modified-site 4..7 /label= Variant residues /note= “The Phe is in the D conformation; Xaa is L-4-chlorophenylalanine; the Trp is in the D conformation; 4 Cys Gly Cys Phe Xaa Tyr Trp Lys Thr Phe Thr 1 5 10 11 amino acids amino acid linear peptide unknown Modified-site 9..11 /label= Protecting group /note= ”Each of the side chain sulfur atoms of the two cysteine residues is protected by an acetamidomethyl group; Modified-site 1..4 /label= Variant residues /note= “The Phe is in the D conformation; Xaa is L-4-chlorophenylalanine; the Trp is in the D conformation; 5 Phe Xaa Tyr Trp Lys Thr Phe Thr Cys Gly Cys 1 5 10 14 amino acids amino acid linear peptide unknown Modified-site 7..10 /label= Variant residues /note= ”The Phe is in the D conformation; Xaa is L-4-chlorophenylalanine; the Trp is in the D conformation; 6 Ala Lys Cys Gly Gly Gly Phe Xaa Tyr Trp Lys Thr Phe Thr 1 5 10 14 amino acids amino acid linear peptide unknown Modified-site 12..14 /label= Protecting group /note= “Each of the side chain sulfur atoms of the two cysteine residues is protected by an acetamidomethyl group; Modified-site 1..4 /label= Variant residues /note= ”The Phe is in the D conformation; Xaa is L-4-chlorophenylalanine; the Trp is in the D conformation; 7 Phe Xaa Tyr Trp Lys Thr Phe Thr Gly Gly Gly Cys Gly Cys 1 5 10 11 amino acids amino acid linear peptide unknown Modified-site 1..3 /label= Protecting group /note= “The N terminus is acetylated; Modified-site 4..7 /label= Variant residues /note= ”The Phe is in the D conformation; Xaa is L-4-chlorophenylalanine; the Trp is in the D conformation; 8 Cys Gly Cys Phe Xaa Tyr Trp Lys Thr Phe Thr 1 5 10 11 amino acids amino acid linear peptide unknown Modified-site 1..4 /label= Variant residues /note= “The Phe is in the D conformation; Xaa is L-4-chlorophenylalanine; the Trp is in the D conformation; 9 Phe Xaa Tyr Trp Lys Thr Phe Thr Cys Gly Cys 1 5 10 8 amino acids amino acid linear peptide unknown Modified-site 1..8 /label= Variant residues /note= ”The Phe is in the D conformation; Xaa at position 4 is 4-chlorophenylalanine; the Trp is in the D conformation; 10 Phe Xaa Tyr Trp Lys Thr Cys Xaa 1 5 9 amino acids amino acid linear peptide unknown Modified-site 1..3 /label= Variant residues /note= “The Lys is linked to a BAT chelator through the side chain nitrogen; Xaa is D-naphthylalanine; the Cys sulfur is methylated; Modified-site 5..8 /label= Variant residues /note= ”The Trp residue is in the D conformation; the Cys side chain sulfur is methylated; 11 Lys Xaa Cys Tyr Trp Lys Val Cys Thr 1 5 14 amino acids amino acid linear peptide unknown Modified-site 1..3 /label= Protecting group /note= “Each of the side chain sulfur atoms Of the two cysteine residues is protected by an acetamidomethyl group; Modified-site 7..10 /label= Variant residues /note= ”The Phe is in the D conformation; Xaa is L-4-chlorophenylalanine; the Trp is in the D conformation; 12 Cys Gly Cys Gly Gly Gly Phe Xaa Tyr Trp Lys Thr Phe Thr 1 5 10 8 amino acids amino acid linear peptide unknown Modified-site 1..4 /label= Variant residues /note= “Xaa is D-naphthylalanine and is linked to DTPA; Trp is in the D conformation; 13 Xaa Cys Tyr Trp Lys Val Cys Thr 1 5 8 amino acids amino acid linear peptide unknown Modified-site 1..4 /label= Variant residues /note= ”Xaa is D-naphthylalanine and is linked to 2-ketogulonyl; Trp is in the D conformation; 14 Xaa Cys Tyr Trp Lys Val Cys Thr 1 5 9 amino acids amino acid linear peptide unknown Modified-site 1..3 /label= Variant residues /note= “Lys is linked to a BAT chelator through the side chain nitrogen and to DTPA at the N-terminus; Xaa is D-naphthylalanine; Modified-site 3..8 /label= Variant residues /note= ”The Trp residue is in the D conformation; each of the Cys side chain sulfur atoms are methylated; 15 Lys Xaa Cys Tyr Trp Lys Val Cys Thr 1 5 9 amino acids amino acid linear peptide unknown Modified-site 2..5 /label= Variant residues /note= “The Phe is in the D conformation; the Trp is in the D conformation; Modified-site 7..9 /label= Variant residues /note= ”The sulfur atom of the cysteine is methylated; the carboxyl group of the C- terminal Thr is reduced to an alcohol; 16 Ala Phe Cys Phe Trp Lys Thr Cys Thr 1 5 8 amino acids amino acid linear peptide unknown Modified-site 1..2 /label= Variant residues /note= “Phe is in the D conformation and is linked to DTPA; Modified-site 1..4 /label= Variant residues /note= ”The Phe is in the D conformation; Xaa is L-4-chlorophenylalanine; the Trp is in the D conformation; Modified-site 7..8 /label= Variant residues /note= “The carboxyl group of the C-terminal Thr is reduced to an alcohol; 17 Phe Gly Tyr Trp Lys Thr Cys Thr 1 5 8 amino acids amino acid linear peptide unknown Modified-site 1..4 /label= Variant residues /note= ”Xaa is naphthylalanine and is linked to DTPA; Trp is in the D conformation; Modified-site 7..8 /label= Variant residues /note= “Lys is linked to a BAT chelator through the side chain nitrogen; 18 Xaa Ser Tyr Trp Lys Val Thr Lys 1 5 8 amino acids amino acid linear peptide unknown Modified-site 1..4 /label= Variant residues /note= ”Xaa is naphthylalanine and is linked to DTPA; Trp is in the D conformation; 19 Xaa Ser Tyr Trp Lys Val Cys Thr 1 5 12 amino acids amino acid linear peptide unknown Modified-site 1..4 /label= Variant residues /note= “The Phe is in the D conformation; Xaa is L-4-chlorophenylalanine; the Trp is in the D conformation; 20 Phe Phe Tyr Trp Lys Thr Phe Thr Gly Gly Cys Lys 1 5 10 11 amino acids amino acid linear peptide unknown Modified-site 1..4 /label= Variant residues /note= ”The Phe is in the D conformation; Xaa is L-4-chlorophenylalanine; the Trp is in the D conformation; Modified-site 9..11 /label= Protecting group /note= “Each of the side chain sulfur atoms of the two cysteine residues is protected by an acetamidomethyl group; 21 Phe Phe Tyr Trp Lys Thr Phe Thr Cys Gly Cys 1 5 10 11 amino acids amino acid linear peptide unknown Modified-site 1..4 /label= Variant residues /note= ”The Phe is in the D conformation; Xaa is L-4-chlorophenylalanine; the Trp is in the D conformation; 22 Phe Phe Tyr Trp Lys Thr Phe Thr Gly Gly Cys 1 5 10 11 amino acids amino acid linear peptide unknown Modified-site 1..4 /label= Variant residues /note= “The first Xaa is D-naphthylalanine and is linked to DTPA; the second Xaa is 4-chlorophenylalanine; Trp is in the D conformation; Modified-site 9..11 /label= Protecting group /note= ”Each of the side chain sulfur atoms of the two cysteine residues is protected by an acetamidomethyl group; 23 Xaa Xaa Tyr Trp Lys Thr Phe Thr Cys Gly Cys 1 5 10 

What is claimed is:
 1. A radiotherapeutic agent comprising: a) a linear peptide having a formula: X¹-A¹A²-B¹B²B³B⁴-C¹C²-X²  wherein X¹ and X² are each independently hydrophilic moieties; A¹, A² and C¹ are each independently a lipophilic D-amino acid, a lipophilic L-amino acid, S-alkylated cysteine, penicillamine, homocysteine, or homohomocysteine; B¹ is D-Phe, L-Phe, D-Tyr, L-Tyr, D-Nal, L-Nal, or Ain; B² is D-Trp or L-Trp; B³ is D-Lys, L-Lys, Hly, Achxa, Amf, Aec, Apc, Aes, or Aps; B⁴ is D-Thr, L-Thr, Ser, Val, Phe, Ile, Abu, Nle, Leu, Nva, or Aib; and C² is D-Thr, L-Thr, Ser, Val, Phe, Ile, Abu, Nle, Leu, Nva, Nal, or Aib; and b) a cytotoxic radioisotope.
 2. The agent of claim 1, wherein X¹ is selected from the group consisting of an amino acid, a peptide containing no more than 10 amino acid residues, a monosaccharide, an oligosaccharide comprising 10 or fewer saccharide units, a poly(N-carboxyalkyl)amine, and a polyoxyanion; and X² is selected from the group consisting of a poly(N-carboxyalkyl)arnine, a polyoxyanion, an amino acid, a peptide containing no more than 10 amino acid residues, a peptide containing no more than 10 amino acid residues and having a carboxyl-terminal amino acid which is reduced to an alcohol, a monosaccharide, and an oligosaccharide comprising 10 or fewer saccharide units.
 3. The agent of claim 1 wherein B¹ is phenylalanine or tyrosine, B² is D-tryptophan, B³ is lysine and B⁴ is threonine or valine.
 4. The agent of claim 1, wherein the radioisotope is selected from the group consisting of iodine-125, iodine-131, astatine-211, scandium-47, copper-67, gallium-72, yttrium-90, samarium-153, gadolinium-159, dysprosium-165, holmium-166, ytterbium-175, lutetium- 177, rhenium-186, rhenium-188, and bismuth-212.
 5. The agent of claim 1, further comprising a non-radioactive metal.
 6. A radiotherapeutic multimer comprising: a) at least two copies of a linear peptide having a formula: X¹-A¹A²-B¹B²B³B⁴-C¹C²-X²  wherein X¹ and X² are each independently hydrophilic moieties; A¹, A² and C¹ are each independently a lipophilic D-amnino acid, a lipophilic L-amino acid, S-alkylated cysteine, penicillamine, homocysteine, or homohomocysteine; B¹ is D-Phe, L-Phe, D-Tyr, L-Tyr, D-Nal, L-Nal, or Ain; B² is D-Trp or L-Trp; B³ is D-Lys, L-Lys, Hly, Achxa, Amf, Aec, Apc, Aes, or Aps; B⁴ is D-Thr, L-Thr, Ser, Val, Phe, Ile, Abu, Nle, Leu, Nva, or Aib; and C² is D-Thr, L-Thr, Ser, Val, Phe, Ile, Abu, Nle, Leu, Nva, Nal, or Aib; and b) a polyvalent linker covalently linked to each of said peptides, wherein the molecular weight of said multimer is less than about 20,000 daltons; and c) a cytotoxic radioisotope.
 7. The multimer of claim 6, wherein the linker is selected from the group consisting of bis-succinimidylmethylether, 4-(2,2-dimethylacetyl)benzoic acid, N-(2-(N′,N′-bis(2-succinimidoethyl)aminoethyl))-N⁶,N⁹-bis(2-methyl-2-mercaptopropyl)-6,9-diazanonanamide, tris(succinimidylethyl)amine tris(2-chloroacetamidoethyl)amine, 1,2-bis-(2-(chloroacetamido) ethoxy)ethane, tris(acetamidoethyl)amine, bis-acetamidomethyl ether, bis-acetamnidoethyl ether, α,ε-bis-acetyllysine, lysine, and 1,8-bis-acetamido-3,6dioxa-octane.
 8. The multimer of claim 6, wherein the radioisotope is selected from the group consisting of iodine-125, iodine-131, astatine-211, scandium-47, copper-67, gallium-72, yttrium-90, samarium-153, gadolinium-159, dysprosium-165, holmium-166, ytterbium-175, lutetium-177, rhenium-186, rhenium-188, and bismuth-212.
 9. A reagent comprising: a) a linear peptide having a formula: X¹-A¹A²-B¹B²B³B⁴-C¹C²-X²  wherein X¹ is H, lower alkyl or substituted alkyl, aryl or substituted aryl, alkanoyl or substituted alkanoyl, aroyl or substituted aroyl, or a hydrophilic moiety not greater than 1500 Daltons in formula weight; A¹, A² and C¹ are each independently a lipophilic D-amino acid, a lipophilic L-amino acid, S-alkylated cysteine, penicillamine, homocysteine, or homohomocysteine; B¹ is D-Phe, L-Phe, D-Tyr, L-Tyr, D-Nal, L-Nal, or Ain; B² is D-Trp or L-Trp; B³ is D-Lys, L-Lys, Hly, Achxa, Amf, Aec, Apc, Aes, or Aps; B⁴ is D-Thr, L-Thr, Ser, Val, Phe, Ile, Abu, Nle, Leu, Nva, or Aib; C² is D-Thr, L-Thr, Ser, Val, Phe, Ile, Abu, Nle, Leu, Nva, Nal, or Aib; X² is —COOR⁹, —CH₂OH, CH₂COOR⁹, or —CON(R⁹)₂, where each R⁹ is independently H, lower linear alkyl, lower cyclic alkyl, or substituted with a hydrophilic moiety; and b) a radiolabel-binding moiety covalently linked to a residue of said peptide other than B¹, B², B³, B⁴ or B⁴.
 10. The reagent of claim 9, wherein the radioisotope-binding moiety has a formula selected from the group consisting of: a) C(pgp)^(s)-(aa)-C(pgp)^(s) wherein (pgp)^(s) is H or a thiol protecting group and (aa) is any α- or β-amino acid; b) a radiolabel-binding moiety comprising a single thiol having a formula: A-CZ(B)-(C(R′R″))_(n)-X  wherein A is H, HOOC, H₂NOC, (amino acid or peptide)-NHOC, (amino acid or peptide)-OOC or R″″; B is H, SH, —NHR′″, —N(R′″)-(amino acid or peptide), or R″″; X is H, SH, —NHR′″, —N(R′″)-(amino acid or peptide) or R″″; Z is H or R″″; R′, R″, R′″ and R″″ are independently H or lower straight or branched chain or cyclic alkyl; n is 0, 1 or 2; and where B is —NHR′″ or —N(R′″)-(amino acid or peptide), X is SH, and n is 1 or 2; where X is —NHR′″ or —N(R′″)-(amino acid or peptide), B is SH, and n is 1 or 2; where B is H or R″″, A is HOOC, H₂NOC, (amino acid or peptide)-NHOC, (amino acid or peptide)-OOC, X is SH, and n is 0 or 1; where A is H or R″″, then where B is SH, X is —NHR′″ or —N(R′″)-(amino acid or peptide) and where X is SH, B is —NHR′″ or —N(R′″)-(amino acid or peptide); where X is H or R″″, A is HOOC, H₂NOC, (amino acid or peptide)-NHOC, (amino acid or peptide)-OOC and B is SH; where Z is methyl, X is methyl, A is HOOC, H₂NOC, (amino acid or peptide)-NHOC, (amino acid or peptide)-OOC, B is SH and n is 0; c)

 wherein X=H or a protecting group; (amino acid)=any amino acid; d)

 wherein X=H or a protecting group; (amino acid)=any amino acid; or e)

 wherein each R is independently H, CH₃ or C₂H₅; each (pgp)^(s) is independently a thiol protecting group or H; m, n and p are independently 2 or 3; A=linear lower alkyl, cyclic lower alkyl, aryl, heterocyclyl, combination thereof, or a substituted derivative thereof; and f)

 wherein each R is independently H, CH₃ or C₂H₅; m, n and p are independently 2 or 3; A=linear lower alkyl, cyclic lower alkyl, aryl, heterocyclyl, a combination thereof or a substituted derivative thereof V=H or —CO-peptide; R′=H or peptide; and wherein when V=H, R′=peptide and when R′=H, V=—CO-peptide.
 11. The reagent of claim 10, wherein the radioisotope-binding moiety has the formula C(pgp)^(s)-(aa)-C(pgp)^(s) and (pgp)^(s) has a formula —CH₂—NH—CO—R wherein R is a lower alkyl having 1 to 6 carbon atoms, 2-pyridyl, 3-pyridyl, 4-pyridyl, phenyl, phenyl substituted with lower alkyl, hydroxy, lower alkoxy, carboxy, or lower alkoxycarbonyl.
 12. The reagent of claim 10, wherein the radioisotope-binding moiety has the formula:


13. A radiotherapeutic multimer comprising: a) at least two copies of a reagent comprising: i) a linear peptide having a formula: X¹-A¹A²-B¹B²B³B⁴-C¹C²-X²  wherein X¹ is H, lower alkyl or substituted alkyl, aryl or substituted aryl, alkanoyl or substituted alkanoyl, aroyl or substituted aroyl, or a hydrophilic moiety not greater than 1500 Daltons in formula weight; A¹, A² and C¹ are each independently a lipophilic D-amino acid, a lipophilic L-amino acid, S-alkylated cysteine, penicillamine, homocysteine, or homohomocysteine; B¹ is D-Phe, L-Phe, D-Tyr, L-Tyr, D-Nal, L-Nal, or Ain; B² is D-Trp or L-Trp; B³ is D-Lys, L-Lys, Hly, Achxa, Amf, Aec, Apc, Aes, or Aps; B⁴ is D-Thr, L-Thr, Ser, Val, Phe, Ile, Abu, Nle, Leu, Nva, or Aib; C² is D-Thr, L-Thr, Ser, Val, Phe, Ile, Abu, Nle, Leu, Nva, Nal, or Aib; X² is —COOR⁹, —CH₂OH, CH₂COOR⁹, or —CON(R⁹)₂, where each R⁹ is independently H, lower linear alkyl, lower cyclic alkyl, a substituted derivative thereof, or substituted with a hydrophilic moiety; and ii) a radiolabel-binding moiety covalently linked to a residue of said peptide other than B¹, B², B³, B⁴ or B⁴; b) a polyvalent linker covalently linked to each of said peptides; wherein the molecular weight of said multimer is less than about 20,000 daltons; and c) a cytotoxic radioisotope.
 14. The multimer of claim 13, wherein the linker is selected from the group consisting of bis-succinimidylmethylether, 4-(2,2-dimethylacetyl)benzoic acid, N-(2-(N′,N′-bis(2-succinimidoethyl)aminoethyl))-N⁶,N⁹-bis(2-methyl-2-mercaptopropyl)-6,9-diazanonanamide, tris(succinimidylethyl)amine tris(2-chloroacetamidoethyl)amine, 1,2-bis-(2-(chloroacetamido) ethoxy)ethane, tris(acetamidoethyl)amine, bis-acetamidomethyl ether, bis-acetamidoethyl ether, α,ε-bis-acetyllysine, lysine, and 1,8-bis-acetamido-3,6dioxa-octane.
 15. The multimer of claim 13, wherein the radioisotope is selected from the group consisting of iodine-125, iodine-131, astatine-211, scandium-47, copper-67, gallium-72, yttrium-90, samarium-153, gadolinium-159, dysprosium-165, holmium-166, ytterbium-175, lutetium-177, rhenium-186, rhenium-188, and bismuth-212.
 16. A reagent having a formula selected from the group consisting of: F_(D)FYW_(D)KTFTGGCK.amide; DDD.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide; Ac.DDD.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide; Hca.G.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide; F_(D)FYW_(D)KTFTC_(Acm)GC_(Acm).amide; F_(D)FYW_(D)KTFTGGC.amide; F_(D)FYW_(D)KTFT(ε-K)GC.amide; (Trc.imide)₂K.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCRR.amide; Trc(Trc.imide)K.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCRR.amide; (Trc.imide)Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCR.amide; F_(D).Cpa.YW_(D)KTFT(ε-K)GCR.amide; K_(D)KKK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKDKD.amide; K_(D)KKK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCDD.amide; (Trc)₂K.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide; Hca.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide; (2-ketogulonyl)F_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide; KKKK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCDDDD.amide; Ac.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide; Ac.KKKKK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide; (2-ketogulonyl)F_(D).Cpa.YW_(D)KTFT(ε-K)GC amide; Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide; DDDD.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKKKK.amide; (DTPA)Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKK.amide; (DTPA)Nal_(D).Cpa.YW_(D)KTFTC_(Acm)GC_(Acm).amide; Ac.KKKKK.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GC.amide; and KDKD.Nal_(D).Cpa.YW_(D)KTFT(ε-K)GCKDKD.amide.
 17. The reagent of claim 16, further comprising a cytotoxic radioisotope.
 18. The reagent of claim 17, wherein the radioisotope is selected from the group consisting of scandium-47, copper-67, gallium-72, yttrium-90, iodine-125, iodine-131, samarium-153, gadolinium-159, dysprosium-165, holmium-166, ytterbium-175, lutetium-177, rhenium-186, rhenium-188, bismuth-212 and astatine-211.
 19. The reagent of claim 16, further comprising a radioisotope selected from the group consisting of gallium-68, technetium-99m, indium-111, and iodine-123.
 20. A kit comprising a sealed vial containing a predetermined quantity of the reagent of claim 16 and a sufficient amount of a reducing agent to label the reagent with technetium-99m, rhenium-186, or rhenium-188.
 21. The kit of claim 15, wherein the reducing agent is selected from the group consisting of a dithionite ion, a stannous ion and a ferrous ion.
 22. A method of labeling the reagent of claim 16 comprising the step of reacting the reagent with technetium-99m in the presence of a reducing agent.
 23. The method of claim 22, wherein the reducing agent is selected from the group consisting of a dithionite ion, a stannous ion and a ferrous ion.
 24. A method of promoting regression of a somatostatin receptor-bearing tumor in a mammal comprising the step of administering an effective therapeutic amount of the agent of claim 1 to the mammal.
 25. A method of promoting regression of a somatostatin receptor-bearing tumor in a mammal comprising the step of administering an effective therapeutic amount of the multimer of claim 6 to the mammal.
 26. A method of promoting regression of a somatostatin receptor-bearing tumor in a mammal comprising the step of administering an effective therapeutic amount of the reagent of claim 9 to the mammal.
 27. A method of promoting regression of a somatostatin receptor-bearing tumor in a mammal comprising the step of administering an effective therapeutic amount of the multimer of claim 13 to the mammal.
 28. A method of promoting regression of a somatostatin receptor-bearing tumor in a mammal comprising the step of administering an effective therapeutic amount of the reagent of claim 16 to the mammal.
 29. A method of promoting regression of a somatostatin receptor-bearing tumor to a mammal comprising the step of administering an effective therapeutic amount of the reagent of claim 17 to the mammal.
 30. A method of reducing hormonal hypersecretion in a mammal comprising the step of administering an effective therapeutic amount of the agent of claim 1 to the mammal.
 31. A method of reducing hormonal hypersecretion in a mammal comprising the step of administering an effective therapeutic amount of the multimer of claim 6 to the mammal.
 32. A method of reducing hormonal hypersecretion in a mammal comprising the step of administering an effective therapeutic amount of the reagent of claim 14 to the mammal.
 33. A method of reducing hormonal hypersecretion in a mammal comprising the step of administering an effective therapeutic amount of the multimer of claim 13 to the mammal.
 34. A method of reducing hormonal hypersecretion in a mammal comprising the step of administering an effective therapeutic amount of the reagent of claim 16 to the mammal.
 35. A method of reducing hormonal hypersecretion in a mammal comprising the step of administering an effective therapeutic amount of the reagent of claim 17 to the mammal.
 36. A method of imaging a somatostatin receptor-bearing site in a mammal comprising the steps of administering a diagnostically effective amount of the reagent of claim 19 the mammal and detecting radioactivity localized at the site.
 37. The method of claim 36, wherein the radioisotope is technetium-99m. 